Optical filter with color enhancement

ABSTRACT

An eyewear lens is described that provides polarization filtering and spectral filtering using polarization interference. The lens produces enhanced saturation and colorfulness, increasing enjoyment when observing commonly encountered imagery. The lens can be configured to optimize accuracy/efficiency when performing a task involving colored imagery, and can improve performance in sports. The lens can further be helpful for color discrimination by those with certain types of color vision deficiency.

This application claims the benefit of U.S. Provisional Application No.62/245,882, filed Oct. 23, 2015, the entire contents of which isincorporated herein by reference.

BACKGROUND

Polarization has become a commonplace functional requirement forreducing uncomfortable glare in eyewear lenses. Spectral filtering usingdyes and dichroic coatings is also commonplace for reducing luminanceand shifting hue, each with performance limitations.

It has been attempted previously to enhance vision with eyewear thatincludes a polarization interference filter, as exemplified in U.S. Pat.No. 7,106,509. From a practical standpoint, the filters disclosedtherein are overly complex with multiple layers of retardation thatcreate multiple output impulses from a polarized input pulse. Thenecessity of the many impulses is to filter in the spectral cyan, thespectral yellow, or both, while efficiently transmitting otherwavelengths. Such an eyewear filter with acceptable angular coloruniformity would require a minimum of six to ten retarder films usingavailable uniaxial materials. For the seven-layer example given therein,the retarder stack would require a minimum of 21 layers, which would beimpractical for an eyewear product.

What is needed, therefore, are improved techniques for spectralfiltering in eyewear lenses.

SUMMARY

The disclosure herein relates to using polarization interferencefiltering to achieve a simple and easily-produced color enhancementfilter. The color enhancement filter may have a series of peaks andvalleys that are selected for color enhancement.

In one aspect, an eyewear lens includes a lens having a spectraltransmission with a sinusoidal shape having a peak at each of the colorsof long-wavelength blue, green, and red and a valley at each of thecolors of high-energy blue, cyan, and yellow/orange.

Long-wavelength blue may be in the wavelength range of 455-480 nm, greenmay be in the wavelength range of 520-555 nm, red may be in thewavelength range of 610-680 nm, high-energy blue may be in thewavelength range of 400-450 nm, cyan may be in the wavelength range of485-515 nm, and yellow/orange may be in the wavelength range of 565-590nm. The transmission at the peaks may be greater than four times thetransmission at the valleys. The transmission at the peaks may begreater than ten times the transmission at the valleys.

In another aspect, an eyewear lens includes a stack of layers, whichinclude a first protective substrate; a polarizer; a retarder stack thatgenerates only two output impulses from a polarized impulse input,wherein the retarder stack provides four to six waves of retardation inthe green wavelength range; a second polarizer; and a second protectivesubstrate.

The retarder stack may have been wide-fielded by including a pair ofmulti-order retarders that are placed with their respective optic axesorthogonal to each other and the pair of multi-order retarders areseparated by one or more zero-order half-wave plates. The retarder stackmay provide five to five and one-half waves of retardation in the greenwavelength range.

In another aspect, a clip-on lens for wearing over a polarized eyewearlens, the clip-on lens includes a stack of layers, including a firstprotective substrate; a first polarizer; a retarder stack that generatesonly two output impulses from a polarized impulse input, wherein theretarder stack provides four to six waves of retardation in the greenwavelength range; and a second protective substrate.

Another aspect relates to an optical filter that includes a polarizationinterference filter (PIF), including at least: an input polarizer; oneor more retarders; and an output polarizer. The PIF has a transmissionacross the 400 nm to 700 nm spectrum that is substantially sinusoidalhaving minimums and maximums, and with a single maximum in the 610 nm to680 nm range, a single maximum in the 520 nm to 555 nm range, and atleast one maximum in the 455 nm to 480 nm range, and with a singleminimum in the 485 nm to 515 nm range, a single minimum in the 565 nm to590 nm range, and at least one minimum in the 400 nm to 450 nm range.

Another aspect relates to an optical filter that includes a polarizationinterference filter (PIF), including at least: an input polarizer; oneor more retarders; and an output polarizer. The PIF has a transmissionacross the 400 nm to 700 nm spectrum that is substantially sinusoidalhaving minimums and maximums, and with a single maximum in the 555 nm to610 nm range, a single maximum in the 400 nm to 480 nm range, and atleast one minimum in the 480 nm to 515 nm range.

The optical filter may provide color enhancement to a person with colorvision deficiency.

Another aspect relates to an optical filter that includes a polarizationinterference filter (PIF), including at least: an input polarizer havingan axis of polarization at a first angle; an input chromatic uniaxialretarder with an optical axis that is offset by +45 degrees from thefirst angle, and having a retardance value of Γ; a pair of zero-orderhave-wave retarders forming an achromatic rotator, with angles ofsubstantially +21.5 degrees and substantially −21.5 degreestherebetween; an output chromatic uniaxial retarder with an optical axisthat is offset by −45 degrees from the first angle, and having aretardance value of Γ; and an output polarizer having an axis ofpolarization at a second angle that is either parallel to orperpendicular to the first angle. The optical filter has a wide field ofview with a Δxy color shift at 30 degrees off-axis of 0.05 or less.

The PIF may have a transmission across the 400 nm to 700 nm spectrumthat is substantially sinusoidal having minimums and maximums, and witha single maximum in the 610 nm to 680 nm range, a single maximum in the520 nm to 555 nm range, and at least one maximum in the 455 nm to 480 nmrange, and with a single minimum in the 485 nm to 515 nm range, a singleminimum in the 565 nm to 590 nm range, and at least one minimum in the400 nm to 450 nm range.

Another aspect relates to an optical filter that includes an inputpolarizer; an input chromatic retarder; an output chromatic retarder; aneutral Polarization Control Unit (PCU) therebetween, including one ormore zero-order half-wave uniaxial retarders, wherein the polarizationtransformation by the PCU creates a net chromatic retardation that issubstantially independent of angle-of-incidence; and an outputpolarizer. The filter may have a transmission across the 400 nm to 700nm spectrum with a single maximum in the 610 nm to 680 nm range, asingle maximum in the 520 nm to 555 nm range, and at least one maximumin the 455 nm to 480 nm range, and with a single minimum in the 485 nmto 515 nm range, a single minimum in the 565 nm to 590 nm range, and atleast one minimum in the 450 nm to 450 nm range.

The input and output polarizers may have parallel axes of polarization,wherein the chromatic retarders are composed of polycarbonate and eachhave a retardance (Γ) of 2.25 waves in the 505 nm to 555 nm range, andwherein the PCU provides rotation about an axis in the range of −19 to−24 degrees and an axis in the range of +19 to +24 degrees. The PCU mayinclude an odd number of half-wave retarders. The PCU may be a singlehalf-wave retarder with the slow and fast axes of the retarder alignedat 0 degrees and 90 degrees with the orientation of a polarization axisof the input polarizer. The PCU may be a compound half-wave retarderwith three retarder layers and with the fast axis of: a first of thethree retarder layers oriented at −30 degrees with an orientation of apolarization axis of the input polarizer, a second of the three retarderlayers oriented at +30 degrees with the orientation of the polarizationaxis of the input polarizer, and a third of the three retarder layersoriented at −30 degrees with the orientation of the polarization axis ofthe input polarizer.

Another aspect relates to an optical filter that includes an inputpolarizer; one or more chromatic retarders; and an analyzing polarizer.At least one of the polarizers has a chromatic polarizing efficiency toincrease the transmission of one or more spectral bands relative to oneor more other spectral bands.

The analyzing polarizer may have a relatively lower polarizingefficiency in the 610 nm to 700 nm range than in the 400 nm to 610 nmrange. The polarizers may have axes of polarization that areperpendicular to each other; and wherein the optical filter furtherincludes a Polarization Control Unit (PCU), including one or morezero-order half-wave uniaxial retarders.

Another aspect relates to an optical filter that includes a polarizationinterference filter (PIF), including at least: an input polarizer; aretarder stack that generates only two output impulses from a polarizedimpulse input; and an output polarizer. The PIF has a transmissionacross the 400 nm to 700 nm spectrum with a single maximum in the 610 nmto 680 nm range, a single maximum in the 520 nm to 555 nm range, and atleast one maximum in the 455 nm to 480 nm range, and with a singleminimum in the 485 nm to 515 nm range, a single minimum in the 565 nm to590 nm range, and at least one minimum in the 400 nm to 450 nm range.

Any combination of any of the following may be added to any of the aboveaspects, and thus may related to PIFs, color enhancement filters, orother optical filters. A substantially neutral white-point may beachieved. Each of the minimums in the transmission spectrum may be lessthan 20% of each of maximums. There may be at least one minimum in the700 nm to 1400 nm range. At least one retarder in the retarder stack mayinclude solvent bonded retarder films. At least one retarder in theretarder stack may include at least one uniaxial stretched retarderfilm. The film may include polycarbonate. The film may includepolyolefin. At least one of the polarizers may include a chromatic dyethat controls a color balance and overall hue of the optical filter.

The optical filter may be arranged as a laminated stack of layers. Thepolarizers may include polyvinyl alcohol. The retarder stack may beformed on glass. The retarder stack may be formed on isotropic plastic.Each of the layers of the laminated stack may lie substantially in theirown plane. Each of the layers of the laminated stack may be curved. Eachof the components of the optical filter may be thermoformed.

The optical filter may be arranged for use by a person having colorvision deficiency. The optical filter may be arranged for use as asunglass lens. The optical filter may be arranged for filtering lightbefore it is captured electronically. The optical filter may be arrangedas part of a display that displays images. The optical filter may bearranged for filtering light provided to a human performing an activity.

Long-wavelength blue may be in the wavelength range of 455-480 nm, greenmay be in the wavelength range of 520-555 nm, red may be in thewavelength range of 610-680 nm, high-energy blue may be in thewavelength range of 400-450 nm, cyan may be in the wavelength range of485-515 nm, and yellow/orange may be in the wavelength range of 565-590nm. The transmission at the peaks may be greater than four times thetransmission at the valleys. The transmission at the peaks may begreater than ten times the transmission at the valleys.

The retarder stack may have been wide-fielded by including a pair ofmulti-order retarders that are placed with their respective optic axesorthogonal to each other and the pair of multi-order retarders areseparated by one or more zero-order half-wave plates. The retarder stackmay provide five to five and one-half waves of retardation in the greenwavelength range.

The PIF may have a transmission across the 400 nm to 700 nm spectrumthat is substantially sinusoidal having minimums and maximums, and witha single maximum in the 610 nm to 680 nm range, a single maximum in the520 nm to 555 nm range, and at least one maximum in the 455 nm to 480 nmrange, and with a single minimum in the 485 nm to 515 nm range, a singleminimum in the 565 nm to 590 nm range, and at least one minimum in the400 nm to 450 nm range.

The input and output polarizers may have parallel axes of polarization,wherein the chromatic retarders are composed of polycarbonate and eachhave a retardance (Γ) of 2.25 waves in the 505 nm to 555 nm range, andwherein the PCU provides rotation about an axis in the range of −19 to−24 degrees and an axis in the range of +19 to +24 degrees. The PCU mayinclude an odd number of half-wave retarders. The PCU may be a singlehalf-wave retarder with the slow and fast axes of the retarder alignedat 0 degrees and 90 degrees with the orientation of a polarization axisof the input polarizer. The PCU may be a compound half-wave retarderwith three retarder layers and with the fast axis of: a first of thethree retarder layers oriented at −30 degrees with an orientation of apolarization axis of the input polarizer, a second of the three retarderlayers oriented at +30 degrees with the orientation of the polarizationaxis of the input polarizer, and a third of the three retarder layersoriented at −30 degrees with the orientation of the polarization axis ofthe input polarizer.

The analyzing polarizer may have a relatively lower polarizingefficiency in the 610 nm to 700 nm range than in the 400 nm to 610 nmrange. The polarizers may have axes of polarization that areperpendicular to each other; and wherein the optical filter furtherincludes a Polarization Control Unit (PCU), including one or morezero-order half-wave uniaxial retarders.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the transmission spectrum provided by an opticalfilter as taught herein.

FIG. 2 is a prior art polarization interference filter (PIF) with aretarder provided between a pair of polarizers.

FIG. 3 is a wide-fielded PIF as taught herein.

FIG. 4 is a wide-fielded PIF with rotator as taught herein.

FIG. 5 includes a pair of graphs that show the spectral distribution ofan image of a pink flower viewed or captured without a filter and viewedor captured with a filter as taught herein.

FIGS. 6a and 6b show the relative shifting of a portion of thewavelength spectrum passed through a long pass filter and without thecolor enhancement filter taught herein (FIG. 6a ) versus with the colorenhancement filter taught herein (FIG. 6b ).

FIG. 7 shows a pair of graphs that illustrate the product of the colorenhancement filter taught herein with each of two different long passfilters, one with a half-power point in the short green and one with ahalf-power point in the long green.

FIGS. 8a and 8b show the relative shifting of a portion of thewavelength spectrum passed through a short pass filter and without thecolor enhancement filter taught herein (FIG. 8a ) versus with the colorenhancement filter taught herein (FIG. 8b ).

FIGS. 9a and 9b show the distribution of color coordinates at differentspectral widths of a bandpass filter as the wavelength spectrum ispassed therethrough and without the color enhancement filter taughtherein (FIG. 9a ) versus with the color enhancement filter taught herein(FIG. 9b ).

FIG. 10 shows a laminate used to make a color enhancement filter astaught herein.

FIG. 11 shows a laminate used to make a filter as taught herein in orderto provide a clip-on lens for a conventional sunglass lens.

FIG. 12 shows a cross-section of a lens including a filter as taughtherein.

FIG. 13 shows the transmission spectrum of a filter as taught hereinwhich might be used by a protanope.

FIG. 14 shows a display system that employs a filter as taught herein.

FIG. 15 shows an imaging system that employs a filter as taught herein.

DETAILED DESCRIPTION

While the embodiments disclosed herein are susceptible to variousmodifications and alternative forms, specific embodiments thereof havebeen shown by way of example in the drawings and are herein described indetail. It should be understood, however, that it is not intended tolimit the invention to the particular form disclosed, but rather, theinvention is to cover all modifications, equivalents, and alternativesof embodiments of the invention as defined by the claims. The disclosureis described with reference to the drawings, wherein like referencenumbers denote substantially similar elements.

Wavelength selective filtration can provide more sophisticated forms ofimage enhancement. The challenges are significant for achievingwavelength selective filtration with angle stability, low stray light,and low cost with many conventional technologies. The inventor seeks toprovide a lens that satisfies the market requirements. Disclosed hereinare techniques and systems related to using polarization interferencefiltering (PIF) to overcome the limitations of prior art eyewear lenstechnologies; to implement spectrally selective filtration necessary for(in particular) optimum outdoor viewing. In certain embodiments, theinvention in-effect modifies the spectrum of the solar illuminant;optimizing it for the human vision system. That is, it transmitsimportant and desirable wavelengths in correct proportions, whilesubstantially attenuating the undesired wavelengths, as established by alarge body of vision research and human factor studies. Undesiredwavelengths can include the high-energy blue, spectral-cyan, andspectral-yellow portions of the spectrum, with desired wavelengthscorresponding to long-wave blue, green, and red portions of the spectrum(in most cases). Benefits of a globally optimized lens includeeye-safety, visual comfort/relaxation, improved mood, full-colorviewing, enhanced chroma, improved visual acuity, and enhanced depthperception.

In one embodiment, a laminate containing one or more retarder films (orphase-difference films) can be used to generate two temporal outputimpulses from a polarized input. The interference of these two impulsesat an output polarizer performs the following filtration:

Provides a transmission minimum in the high-energy blue portion of thespectrum (e.g., 400-450 nm).

Provides a transmission minimum in the spectral-cyan portion of thespectrum (e.g., 485-515 nm).

Provides a transmission minimum in the spectral-yellow portion of thespectrum (e.g., 565-590 nm).

Provides a transmission maximum in the long-blue portion of the spectrum(e.g., 455-480 nm).

Provides a transmission maximum in the green portion of the spectrum(e.g., 520-555 nm).

Provides a transmission maximum in the red portion of the spectrum(e.g., 610-680 nm).

It should be understood that the wavelength ranges listed above areapproximate, and deviations of 5-10 nm from the ranges may also beacceptable.

The two-impulse PIF approach has the benefit of simplicity, in that itenables multiple pass-bands and stop-bands in the visible spectrum witha simple structure. Accordingly, these features represent singlewavelength maxima/minima in each of the above bands as illustrated inFIG. 1, as opposed to prior art filtration characterized by flatregions, or multiple maxima/minima per band, which call for an increasednumber of impulses. In certain embodiments, the minima are fairly highin optical density, with preferred transmission values of <2% of peaktransmission at center wavelengths for a filter with a high degree ofcolor enhancement. The techniques disclosed herein rely upon theperiodic nature of two-beam (two-impulse) interference, with theretardation parameter optimized to place maxima and minima optimally, inaccordance with the above requirements.

Further, embodiments are shown which preserve the on-axis(normal-incidence) filtering characteristics over a broad range ofincident cone angles. Such spectral stability is essential to ensuringthat optimum viewing enhancement and color uniformity are maintainedover the entire field of view. A benefit of the PIF approach is thatstructures which preserve relative amplitude and path-length differenceare feasible, with the functional benefit of spectral stabilityoff-normal. One embodiment uses four retarder layers to achievenormal-incidence performance virtually identical to a single retarderlayer, but which is also preserved (to within a just noticeabledifference) to an angle of 30 degrees.

Eyewear lenses are described which combine a PIF for relatively highspectral gradient (HSG), or steep transition slope filtration forenhanced chroma, with a filter of low spectral gradient (LSG) filtration(e.g. a chromatic polarizer) to create a globally optimized lens. Chromaenhancement can include increased image colorfullness and/or increasedsaturation. The HSG filter can pass the three additive primary colors(red, green and long-wave blue) with a quasi-neutral white point, whilethe LSG filter can be utilized to establish the relative level of each;as needed to give the optimum viewing experience for a particularenvironment and/or activity. As such, the HSG filter can be used as abuilding block for lenses of any white-point hue.

Practical manufacturing methods are described for creating functionalsheet-stock and lens blanks which are robust and conform substantiallyto conventional processes and materials used in the industry. Laminatescan include other functional materials, such as photo-chromics, variousLSG filters to shape color balance (which may be included in thepolarizer function), UV/IR filters, and auxiliary high-energyblue-blocking filters. Such sheets can be cut and fixtured into variousmechanical mounts (e.g. frames) as required for flat filter product, orsingle-axis curved product. The laminates can be fabricated which areflat, or with single-axis curvature when in a relaxed state. Theaforementioned sheet stock can also be thermoformed to producecompound-curved product (e.g. spherical or toroidal). Such lenses canalso receive external functional coatings that are known in the art,including antireflection, hydro-phobic, oleo-phobic, gradient/uniformmetallic reflective coatings, dielectric mirrors, etc.

The techniques taught herein relate to filters and lenses, particularlypolymer plano/prescription eyewear lenses, incorporating polarizationinterference to provide both polarization and spectral filtering.Retarder materials are used to induce a wavelength dependent phase shiftfor spectral filtering that, among other functions, enhances thesaturation and colorfulness of an observed scene.

A two-beam PIF 20, shown in FIG. 2, includes a retarder 22 of retardanceΓ between an input polarizer 24 and an output polarizer 26. In typicalPIF configurations, a uniaxial retarder is oriented at ±45 degreesbetween parallel (or sometimes crossed) linear polarizers. Parallelpolarizer configurations form the building blocks of traditional LyotPIFs. The transmission (T) of a single-stage filter is given byT(λ)=T ₀(λ)cos²[Γ(λ)/2], where T _(o) is the maximum transmission value

Where the phase difference, or retardation is determined by the opticalpath-length difference between the equal-amplitude eigen-modes, or,Γ(λ)=2π[n _(e)(λ)−n _(o)(λ)]

and T₀(λ) is an envelope function that can account for other sources ofattenuation, including (chromatic) polarizer absorption. In addition tothe inverse-wavelength dependence, the retardation also depends upon thedispersion of the birefringence, which is the refractive indexdifference between the eigen-modes.Δn(λ)=n _(e)(λ)−n _(o)(λ), wherein n _(e) is the index of refraction inthe extraordinary axis and n _(o) is the index of refraction in theordinary axis

Because the simple PIF represents two-beam (or two impulse) interferenceof equal-amplitude, the transmission function oscillates with unityamplitude and frequency determined by the “order” (number of waves ofphase-difference) of the retarder, as shown in FIG. 1. Historically,filter units were cascaded to create high resolution (multi-impulse)band-pass filters using polished inorganic crystals (e.g. quartz). Inrecent years, organic films manufactured for the display industry havebeen used to create PIFs with larger aperture, thinness, and lower cost.

Most plastics exhibit birefringence as-fabricated, though films that aremass-manufactured to have engineered retardation are generally used inthe display industry and are composed of either polycarbonate (PC), orpolyolefin (with trade-names cyclic-olefin copolymer (COC), orcyclic-olefin polymer (COP)) resins. Other materials have also been usedto make retarder films, some of which are used in the display industryas compensation films. These include polyvinyl alcohol (PVA), cellulosedi-acetate, Transfan, and various liquid-crystal polymers (manufacturedby e.g. Nippon Oil, Fuji, and Rolic). Some materials provide the desiredoptical characteristics as fabricated (e.g. cast, extruded, or coated).But more typically, a quasi-isotropic substrate is heated and stretched(usually machine-direction (MD), but also transverse direction (TD), oreven diagonally), orienting molecules and producing a desiredretardation value. For both PC and olefin resins, the refractive indexin the stretching direction (extraordinary axis) increases, sostretching an isotropic substrate in a single direction creates apositive uniaxial material.

Polycarbonate (PC) is characterized as providing a large range ofretardation values (100 nm to over 10,000 nm), has a relatively highvisible birefringence dispersion, is more sensitive to mechanicalloading (in terms of altering the as-fabricated retardation value andoptic axis orientation), is high in refractive index (typically about1.6), and moderately low in haze. PC is a good substrate mechanically,and environmentally stable, though it tends to swell when exposed tomoisture. Products have been developed involving uniaxial stretching,biaxial in-plane stretching, and even stretching in the thicknessdirection. The latter is important for making single films that have astable retardation and amplitude splitting as a function of incidenceangle. PC is regularly used as a substrate in the eyewear industry, isthermo-formable, and can accept functional coatings, which are oftenrequired. Display PC films are typically fabricated in the 50-100 μmthickness range, though arbitrary thicknesses are manufacturable.Historically, manufacturers have included (e.g.) Teijin, Kaneka, NittoDenko and Sumitomo.

COP is characterized as providing a smaller range of retardation values(typically 50 nm to 300 nm), has a relatively low visible birefringencedispersion, is insensitive to mechanical loading (in terms of alteringthe as-fabricated retardation value and optic axis orientation), ismoderate in refractive index (typically about 1.5), and low in haze. COPfilms with higher retardation values (<700 nm) have also beendemonstrated. Products have been developed involving uniaxialstretching, and biaxial in-plane stretching. As of the date of thisdisclosure, thickness-direction stretching of COP is not available, andtherefore neither is the feature of providing a stable retardation andamplitude splitting as a function of incidence angle. COP is not asregularly used as a substrate in the eyewear industry, but it isthermo-formable. Progress is being made to identify processes that allowCOP to accept functional coatings. Display COP films are typicallyfabricated in the 20-100 μm thickness range, though arbitrarythicknesses are manufacturable. Manufacturers include Zeon and JSR.

The retarder(s) may supply sufficient retardation to meet the filtrationrequirements described previously. This requires several waves ofvisible retardation; e.g., four to six waves with PC. So while a singlePC film can meet the requirements at least on-axis, COP currentlycannot. And even a single PC film with sufficient retardation iscurrently considered exotic and more difficult to source. As such, it isanticipated that products with either material are likely to requirelamination of two or more films to meet the filtration requirements,even at normal incidence. Accordingly, high retardation films can bemanufacturing using roll-to-roll lamination processes, in particularusing solvent bonding.

The other technology required to implement a PIF is the polarizer. Likethe retarder, early versions of PIFs utilized polished inorganiccrystals. Today, any PIFs of significant aperture size utilize organicfilms. The functional layer is a polyvinyl alcohol (PVA) substrate,typically about 25 μm thick, which is stretched to orient a long-chainorganic dye. Invented by the Polaroid Corporation, such films are todaymanufactured by companies that support the display industry. While theiodine polarizer is the workhorse of the direct-view display industry(owing to high polarizing efficiency (PE), high transmission, andneutral color), dye-stuff polarizers are attractive for the eyewearindustry due to their tolerance to high temperatures. Dye polarizers canbe manufactured which have fairly arbitrary transmission level and hue,with sufficient PE, as needed for eyewear lenses. Polatechno and Sanritzhave manufactured PVA polarizers for the sunglass industry. There arealso several manufacturers of sunglass polarizer in Taiwan and China.

Two polarizers may be used. The input polarizer polarizes the incomingradiation, substantially eliminating specular glare from a scene, whileproviding the polarized input necessary to implement the PIF. The secondpolarizer analyzes the phase difference induced by the retarder(s).These polarizers can have very different characteristics depending uponthe functional requirements of the lens. Because the input polarizer isused to eliminate specular glare, it has a horizontal absorption axisand tends to provide moderately high PE throughout the visible spectrum.It may have a neutral appearance, but it may be significantly chromaticdepending upon the desired hue of the lens and the color of theanalyzing polarizer. The analyzing polarizer should have a moderatelyhigh PE at any wavelength for which polarization interference is usedfor filtration. However, it need not have a moderately high PE forwavelengths requiring sustained high transmission. Moreover, it mayactually be desirable to have spectral regions of low PE as a means ofspoiling the PIF function. Specifically, the PIF could provide anoscillatory spectrum over a portion of the visible band, with anotherportion of the visible band being free of oscillation. The latter may beneeded to maintaining high transmission over a particular extended band.

In principle, the filter can be implemented using any polarizerexhibiting the necessary functional requirements, many of which areknown in the prior art. This includes absorptive (e.g. dye stuff, oriodine) polarizer, or reflective polarizers. Examples of the latterinclude e.g. wire-grid polarizer from Moxtek or Asahi-Kasei, DBEF from3M, and cholesteric LC polarizers from Chelix. In some instances, thepolarizers are neutral, relying upon auxiliary filters to establish thelens efficiency and hue. In other cases, either one or both of thepolarizers are chromatic, and have a dual purpose. In the case ofcompound-curved lenses, the polarizer must be durable enough towithstand the thermo-forming process. For reasons of opticalperformance, cost, thickness, manufacturability, durability, andavailability, the PVA polarizer film tends to be the most attractive.

The PIF and polarizer spectra (as well as other functional filterlayers) can be used in a complementary fashion to determine the desiredcomposite spectral response. The appearance of a lens associated withthe PIF spectrum can be quasi-neutral (i.e. assuming that the polarizersare neutral). That is, a white object can be observed to haveinsignificant shift in hue or color-temperature if so desired. Whenconsidering only the oscillatory characteristic of the PIF, the whitepoint can be substantially preserved by proper selection of retardermaterial, retardation value, and polarizer orientation. Conversely, inthe event that the oscillatory characteristic of the PIF causes anundesired shift in the white point (e.g. excessive yellow filtrationcausing a blue shift in hue), fidelity of the white-point can berestored via LSG filtration (e.g. relatively high absorption of blue ina polarizer). Thus, we describe the combination of relative HSGfiltering from the PIF, with LSG polarizer absorption, to achieve thedesired overall result. If desired, the LSG filter can perform a “colorbalance” function on the three primary bands established by the PIF. Oralternatively, it can be used to emphasize the brightness of aparticular primary color over the remaining primary colors.

The shift in the filter white point can be calculated by (e.g.)measuring the color coordinates of a standard white illuminant scatteredfrom a Lambertian (e.g. Spectralon) target, and comparing with the colorcoordinates obtained with the filter inserted. A neutral filter functionis one that preserves the color coordinates of substantially neutralobjects. It includes the entire set of filter transmission spectra thatwould produce substantially identical color matches when compared to theunfiltered light with a suitable adjustment in luminance. The presentinvention seeks to identify a subset of filter functions thataccomplishes this while increasing the colorfulness of a set ofnon-neutral objects.

Techniques for implementing tri-stimulus (or multi-notch) filtering aretypically either of reflection or absorption based technologies.Intrinsically reflection-based filters include (e.g.) dielectric stacks,cholesteric LCs, Rugate filters, and Fabry-Perot structures.Absorption-based filters include (for example) organic dyes,photochromics, certain liquid crystals, and rare-earth doped materials.Polarization interference filters (PIFs) are typically absorption-based(i.e. the exit polarizer absorbs the rejected portion of the spectrum),though they can be implemented with reflection-based polarizers, asdiscussed previously. The above technologies have strengths andweaknesses when employed in eyewear filters, though it will be shownthat the PIF can be particularly well matched to the presentrequirements.

A characteristic of any interference filter is that the spectral profile(i.e. transmission spectrum) is intimately coupled with the phaserelationship between two or more interacting waves. In the case ofisotropic interference filters, such as synthetic dielectric stacks, thespectral profile is changed, or is distorted when light passes throughthe structure off-normal. If the structure is isotropic, then thischange in profile is independent of azimuth, and only depends uponincidence angle. In the case of anisotropic interference filters, thetransmission function can have azimuth dependence. The details of thestructure determine the nature of the spectral change. Certain isotropicinterference filter designs exhibit a substantially pure blue-shift ofthe normal-incidence spectral profile. Other designs may show a loss inoptical density (or breakup) of the desired notch spectrum, distortionsof the pass-bands, or both. In any case, such phenomena manifestthemselves as a perceptible angle-dependent change in the appearance ofthe image (i.e. brightness, hue, and chroma shifts) when used as aneyewear lens. Interference filter designs with high spectral gradients(HSG), or steep transition slopes, and high optical density can beparticularly problematic, as the spectral distortions are often moresevere, and thus more perceptible. Shifts which are noticeable withinthe field of view can be visually objectionable, and can even render alens technology/design unacceptable.

In the case of a two-impulse (anisotropic) PIF, shown in FIG. 2, thespectral distortion is a function of both azimuth and incidence angles.These spectral distortions tend to be associated with pure shifts in thesinusoidal profile. In the 0/90 degree azimuth (plane of incidence (POI)containing the polarizer absorption axis, or orthogonal to it), suchshifts are relatively insignificant. Assuming a positive-uniaxialmaterial, a significant blue-shift occurs when the POI contains theoptic axis. Conversely, a significant red-shift occurs when the POI isorthogonal to the optic axis. Again, such spectral shifts may benoticeable within the visual FOV of an eyewear lens, and thereforeunacceptable.

To address these artifacts, a uniaxial retarder 30 can be “wide-fielded”using the configuration shown in FIG. 3. Specifically, the retarder isdivided in half (to create a pair of retarders 32 and 34), the opticaxes are crossed, and a half-wave (HW) retarder 36 is insertedthere-between with optic axis parallel/perpendicular to the axes of thepolarizers 38 and 40. Functionally, the half-wave retarder 36 reflectsthe state of polarization (SOP) about the polarizer absorption axis andswitches the handedness of the SOP. This has the effect of making theretarders 32 and 34 appear to have parallel optic axes (and thereforeadditive retardation). However, the additional incremental retardationaccumulated off-normal from the first retarder 32 is of opposite signand substantially the same magnitude as that accumulated in the secondretarder 34, again due to the action of the HW retarder 36. As such, theadditional net retardation accumulated off-normal from the pair ofretarders 32 and 34 is substantially nullified. This greatly stabilizesthe filter spectral output with angle, such that the profile isvirtually as stable in the ±45 degrees azimuth as it is in the 0/90degrees azimuth.

While the wide-field retarder 30 includes three retardation films, thedesired impulse response degenerates to that of a single retarder, ortwo impulses (N=2). Note that N represents the number of impulsesherein, where it represents the number of retarders (giving N+1impulses) in some of the prior art. To the extent that the HWretardation is stable with wavelength and incidence angle (δ=0), thespectral profile remains stable. However, if the retardation is afunction of wavelength, incidence angle, or both (the latter being mostcommon), the spectrum becomes unstable, as would the brightness, hue andchroma of a lens. The instability can manifest itself as a shift inspectral profile, a loss in optical density of the null, and a loss inpeak transmission. While the chromatic nature of the HW can causefailure of the N=2 objective, it is not a design parameter leveraged toenhance the quality of the spectral profile by increasing the number ofimpulses. Rather, it has an undesired impact on performance that isideally mitigated using additional half-wave retarders to restore theN=2 objective.

The function of the three-layer wide-field structure described above canbe accomplished in a single film using a biaxially stretched retarder(e.g. NRZ product developed by Nitto Denko). Specifically, when the filmis stretched in the thickness direction such that the resultingrefractive index is intermediate between the in-plane refractiveindexes, the spectral shift off-normal is diminished. When it is nearthe in-plane mean, the spectral profile becomes very stable withincidence angle at any azimuth. In instances where multi-layer laminatesare required to produce multi-impulse (N≥3) retarder-stack filters(RSFs) with more sophisticated spectral profiles, this is attractive inthat there is no need for additional layers to wide-field the structure.It should be noted that there is a range of refractive index in thethickness direction that is achievable in manufacturing, which iscoupled with the in-plane retardation. Stated differently, the conditionfor optimum angle insensitivity of retardation is tied to a specificrange of in-plane retardation values, which can be limiting. Therefore,it can be challenging to fabricate the high retardation values desired,with optimum thickness-direction retardation. Such biaxial films havebeen supplied to the display industry using PC, though they are againexotic and more expensive than their uniaxial counterparts. As of thedate of this disclosure, at least one major manufacturer has ceasedproduction due to lack of volume demand.

In the absence of the biaxial retarder film described above, the numberof films required to implement a wide-fielded RSF is tripled. Accordingto a prior art example, a wide-fielded RSF with seven fundamental layers(N=8) would then require 21 layers. Moreover, it is preferred that thehalf-wave retarder perform the above described polarization manipulationsatisfactorily throughout the spectral range. The aforementioned may beneeded in order to achieve high-density minima and maintain high peaktransmission throughout the visible, and to maintain that performancewith incidence angle. Because the spectral range is broad for thepresent invention (i.e. the entire visible spectrum), a single-layerzero-order half-wave retarder may not be sufficiently achromatic. Thisis particularly so when using PC, due to its relatively highbirefringence dispersion. However, there are single-layer achromaticretarders, such as the WB140 product from Teijin (used for e.g. OLEDcircular polarizers) that can potentially solve this problem. Awide-band single-layer achromatic half-wave of this type could be usedin the present invention to produce a three-layer wide-fielded filter.

In the absence of a single-layer achromatic HW film, compound HWretarders of the prior art (three films per half-wave) may be required,which would require a minimum of five layers per wide-fielded retarder.In that case, an RSF with seven fundamental layers would then require 35layers for a wide-fielded version. An eyewear filter with this manylayers would be relatively expensive to manufacture, and perhapsimpractically thick, and/or difficult to thermo-form.

Eyewear RSF designs of the prior art employ additional impulses (N>2) toachieve more sophisticated spectral profiles than possible withtwo-impulse PIF designs. However, they are relatively complex and thusmore expensive to manufacture. A simple two-impulse design withcarefully selected retardation characteristics can be used to attain thedesired filtration operation. Additionally, a wide-fielded version ofthis filter with a practical number of layers can be implemented foruniform filtering with incidence angle. Again, even though thesestructures contain a minimum of three retarders, the filter function isin general characterized as having two-impulse interference.

In addition to the characteristics that determine the spectral andangular behavior of interference filters described previously, are thosethat influence stray-light. Dielectric-stack filters utilize (typicallyabrupt) steps in refractive index along the normal direction in order tocreate multiple interacting waves. The complex superposition (i.e.amplitude and phase relationship) of these waves at a particularwavelength determines the extent to which the transmitted/reflectedwaves are reinforced. Thus, the goal is to design a stack thatreinforces the transmission of the desired wavelengths, while stronglyreflecting the undesired wavelengths. For instance, filters exhibitingHSG (steep transition slopes), high density notching over extendedbands, and flat-top pass-bands are routinely manufactured. Such is thecase with eyewear filters used for “6-primary” spectral-based 3D cinema.However, because the rejected band is reflected, there is a much greateropportunity for said light to contribute to stray light than were itabsorbed. And insofar as the rejected portion of the light can comprisea significant proportion of lumens to that transmitted, there is apotential for significant stray light that creates eyestrain anddiscomfort. Consider the significant impact of using an antireflectioncoating, which is designed to minimize only the 4% of light reflectedfrom an uncoated surface, whereas a dielectric stack lens may be calledupon to reflect well over 50% of the incident lumens.

For light directly incident on the back surface of an eyewear lens(viewer side), coated with a dielectric stack filter, the level ofreflection can be very strong. Some light shielding can be provided bythe temple design which mitigates this, but it limits frame stylechoices, and is seldom completely effective. Additionally, lighttransmitted through the lens is scattered from the viewer into a broadrange of angles; creating an image of the viewer at roughly the reliefdistance (distance from eye to lens) in front of the lens. Thisghost-image competes with the transmitted image to create eye fatigueand discomfort. The angle sensitivity of a reflection interferencefilter spectrum exacerbates the situation, since light efficientlytransmitted through the lens initially may not be so when returned tothe lens at a different angle. Again, decoupling filtration fromreflection not only allows much reduced lens reflection, but it alsoallows antireflection coatings on either surface, which can furtherenhance the see-through of the lens. In general, the preferred lenscompletely extinguishes any stray light by using a low-haze, lowfluorescence, absorption-based filter in combination with low externalreflection (i.e. AR coatings).

The lens/filter disclosed herein can be relatively free of stray-lightissues, because it is an absorption-based filter. For light directlyincident on the back surface, the 4% reflection can be greatlydiminished via an AR coating. Another source of stray light is thattransmitted through the lens, scattered from the viewer, and reflectedfrom both surfaces. Again, a back-surface AR coating can greatlydiminish the contribution of that surface. Light reflected from thefront surface must pass through the absorptive filter twice. So inaddition to viewer scattering efficiency (albedo) and the 4%reflectivity of an uncoated front surface, there is a double-passattenuation of the composite filter. As a ratio of the initialtransmitted image, this level can be very weak. For example, assuming atypical 15% photopic-transmission absorptive filter, an uncoated frontsurface would return a small fraction of a percent of the luminanceassociated with the intended image. As such, an AR coating on the back(inner) surface is of much greater importance in reducing stray lightthan an AR coating on the front surface. Note that absorptive filterscan make reflective coatings on the input face viable (i.e. from astray-light standpoint) due again to the low double-pass efficiency ofthe stray light.

The perceived lightness, hue, and chroma of an object observed through alens are dependent upon the power spectrum and angular distribution ofthe illuminant (e.g. the sun), the spectral reflectance (surface andvolume scatter) of the object, the spectral transmission of the eyewearfilter, and of course the response of the human vision system. Anexhaustive analysis of this requires use of the bi-directionalreflectance distribution function (BRDF) to describe the interaction oflight with objects. And the perception of objects in a scene does notconsider them in isolation, but requires context due to the spatialdependence of the vision system. It is a highly complex problem toanalyze thoroughly. But for the present purposes, an eyewear filter canbe thought of as a (relatively practical) means of modifying the solarilluminant; to make it a “better” light source for viewing the world.Safety objectives notwithstanding, there are several reasons for doingthis many of which are familiar.

When the sun is too intense, a sunglass lens reduces the brightness to acomfortable level; a filtering operation that can be spectrally neutral,or flat. A filter can also effectively change the color temperature ofthe sun, which can shift the hue of all objects in a manner that is morecomfortable and/or pleasant to the eyes. Such filtering can beaccomplished via small gradients of low absorption density in thetransmission spectrum. For example, a brown lens can be a quasi-linearspectral ramp, with transmission increasing throughout the visible.Color constancy can make such hue shifts acceptable/desirable to thevision system, but there are limits: High density attenuation spanningany of the prime colors (associated with the S, M and L cone receptors,and subsequent processing that determine the perception of color) isundesirable, as it greatly diminishes color rendering and appreciationof full-color in images. For instance, a “blue-blocker” lens whichabsorbs all light in the 400-500 nm band substantially eliminates anyS-cone excitation, creating a saturated yellow appearance, andeliminating any ability to appreciate blue content in a scene. It canalso affect peripheral vision, since the density of S-cones increasesoutside of the fovea. Conversely, the lens designs taught herein providefor regions of high-density blocking which still permit full-colorperception, while making the world more pleasing, engaging, andcomfortable to observe.

In addition to conventional sunglass filtration, there is mountingevidence that humans prefer a more colorful world than they observe withthe naked eye. Such filter functions require more selective spectralgradients than discussed previously. Examples of preference for enhancedchroma are as follows: Photographic film that increases chroma has longbeen known to be a favorite among artists for landscape photography(e.g. Fuji Velvia), consumers typically increase chroma to unnaturallevels when processing their digitally captured images, moderntelevisions (OLED and quantum-dot backlight) are frequently demonstratedwith scenes containing unnaturally “punched-up” colors, and researchersdesigning solid-state lighting have found that consumers preferwavelength selective sources that increase chroma. This is even the casewhen observing challenging imagery, such as skin tones. Recent humanfactor studies have verified that people often prefer images withenhanced chroma over those that better preserve color fidelity.Preference for higher chroma, and the benefits of trichromaticillumination to visual acuity, has been known for some time; Thorntonobserved this when designing fluorescent lighting. It therefore seemsquite reasonable to expect that consumers would also prefer to view thenatural world with increased colorfulness and saturation, via filtersthat create a more wavelength selective illuminant. In performing recenthuman-factors testing, researchers have also come to recognize thatindustry standards such as correlated color temperature (CCT) and thecolor rendering index (CRI) do not adequately capture viewer preferencefor chroma enhancement, with proposed new standards currently underconsideration.

In addition, it should be pointed out that polarizers are incorporatedto perform an object-dependent filtering that is complementary tospectral filtration. Functionally, a polarizer can be considered afilter that selectively attenuates the lightness of specific objects ina scene; usually those in greatest need of it for visual comfort.Eliminating glare allows more comfortable viewing of directlyilluminated objects with high surface scatter, facilitating observationof the volume scatter more associated with the color of the object. Thisform of filtration is unique, in the sense that it depends upon thegeometry of the scene (e.g. the viewer position, the position of theilluminant, and the orientation of specular objects relative to each).

Maintaining color fidelity of a scene, often associated with the CRI isin many instances at odds with the objectives of the present invention.That is, the lens often deliberately distorts the hue of objects inorder to achieve the goal of enhancing the colorfulness and/orsaturation. For instance, objects that appear to be pastel brown oryellow-green to the naked eye, can appear to have increased chroma withhue shifted toward the red, or green, respectively, when viewed throughthe filter. A consequence of this distortion when observing the naturalworld is that boundaries associated with objects in juxtaposition thatotherwise appear quasi-homogeneous, can become more evident. Forinstance, a scene of dense vegetation may have a variety of species oftrees and shrubs randomly positioned in space. Without the filter, thescene can appear drab; to possess little contrast and color diversity.As such, the vegetation can tend to blend together and appear flat. Thefilter can emphasize subtle differences in spectral reflectance (e.g.the specific color of leaves), as well as differences in structure (e.g.the specific distribution of branches and leaves within a tree). Theseamplified chroma-steps (and to some extent luminance-steps), canfacilitate identification of boundaries of objects andgroupings/clusters of structure/texture, helping the vision systembetter locate them in space. Thus, in addition to making the scene moreinteresting and compelling, the filter of the present invention alsoprovides a better sense of depth perception.

Further to this, the lenses taught herein can enhance athleticperformance and enjoyment of outdoor activities. This can result fromimproved visual acuity and depth perception, as well as thepsychological benefits owing to the positive emotional response obtainedwhen viewing a more stimulating image. The tendency is for scenerycontaining enhanced color to appear more vibrant and stimulating,increasing a sense of well-being. A chroma enhancing lens can be used bygolfers to better assess the spatial location of trees (and otherobjects), to assess subtle differences in the characteristics of grass,to visually locate the ball, and simply to better enjoy the setting.Other outdoor sports requiring rapid detection of moving objects (e.g. aball or competitor) on a particular background can benefit from thetechniques taught herein. They can enhance visual edge-detection andimprove response-time. Fishermen can use the combined polarization andspectral filtration of the present invention to better observebelow-water activity. Cyclists, hikers, runners, andsnorkelers/scuba-divers can all improve upon the quality of theirexperience by wearing eyewear using the present invention.

Commercial/Military applications can also benefit from the improvementsherein by making it feasible to complete tasks involving chromaticobjects/subjects faster, and with better precision. Those working in theagriculture industry can more easily detect the health of vegetation, orlocate fruit/vegetables when harvesting. Pilots and military personnelcan also benefit from the characteristics of the present invention formore accurately/rapidly extracting information from a scene. Anytechnician/professional engaged in a task involving manipulation orassessment of content that uses color vision can potentially improveupon efficiency and accuracy using the present invention.

The lenses taught herein can also be useful for improving the perceptionof color among the large population of humans (particularly males)afflicted with color vision deficiency (CVD). Through appropriatenotching (e.g., of anti-prime colors spectral cyan and yellow), thefilter can greatly influence the opponent signals that determiner colorperception.

Customization of the present invention for a particularactivity/environment can also be accomplished. Lens design variablesinclude selection of retarder order, retarder birefringence dispersion,adjustments in green center wavelength, polarizer configuration (anglesof input/output polarizers), polarizer type (e.g. neutral, or colored),spectral characteristics of the polarizing efficiency, and auxiliaryfiltration. The objective may be to better detect an object on aparticular background, (e.g. such as a tennis ball on the courtbackground color). The optimization entails identification of a lenstransmission spectrum which maximizes the perceived contrast between thesalient feature(s) in the image and the background, including thecombined parameters of hue, chroma and lightness (brightness). Thelatter can be quantified using a number of standard techniquescharacterizing visual response, such as L*a*b* space, HSV, HLS, and thelike as are well known in the art.

The visual impact of the filter on the spectral signature of the inputis conveniently done by considering specific chromatic objects. Theretina normally contains three types of color photo receptors; oftentermed short (S), medium (M), and long (L) wavelength cones, which areprocessed locally to create sum/difference signals. In order for anobject to appear saturated yellow, for example, it must have a strong(M+L) signal relative to the blue (S) opponent signal. An object thatefficiently scatters all but blue wavelengths can thus have both a highdegree of yellowness and a high degree of lightness, owing to the largepotential (M+L) signal. For instance, yellow turned leaves on a treeusually appear much lighter than surrounding green leaves, and it isthis lightness that contributes substantially to the beauty of fallcolors. Such broad-band (e.g., yellow) objects are much more common inthe natural world than narrow-band (e.g., yellow) objects. Further, anyspectral signature (i.e. illuminant spectrum multiplied by reflectancespectrum) with a relatively weak S-cone signal and a balanced red/greenopponent signal represents a metamer of yellow.

Consider such a broadband saturated yellow object filtered by a lens astaught herein. The suppression of a narrow band of wavelengths centeredon unique yellow (about 577 nm) can diminish perceived lightnesssignificantly, while having little impact on hue and chroma.Accordingly, such a reduction in lightness of broadband yellow objectscan affect balance relative to other objects of a different spectralreflectance that are less influenced by the notch filter. For instance,a reddish object may experience much lower attenuation by a filter thana juxtaposed yellow object, thereby affecting their relative balance oflightness. The notch can also have significant impact on the hue andchroma of the reddish object, making it shift toward a more saturatedred. The relative lightness and increased chroma of the reddish object,versus the filtered yellow object can create a striking and pleasingvisual quality.

For the naked eye, the red-green opponent signal is determined by thedifference between overlap integrals associated with the product of Land M curves, respectively, with the spectral signature of an object. Byattenuating a band between primary red and green, long-wave green (orgreenish yellows), and yellow/orange wavelengths can have relativelylittle influence on the red/green opponent signal, and therefore theperceived hue. Equivalent to changing the illuminant spectrum discussedpreviously, it is as if the filter modifies the color matching functionsof the eye. The range of wavelengths over which the M and L conescollect photons becomes confined substantially to primary green andprimary red wavelengths. In the absence of information centeredapproximately on unique yellow, the ability to detect hue in this partof the spectrum relies predominantly on the relative amplitude ofprimary green to primary red, as defined by the band-edges of the notchfilter.

The ability of the vision system to detect small shifts of aquasi-monochromatic input is heightened in the cyan and yellow portionsof the spectrum. In the yellow, this is where a differential change inthe red-green opponent signal has the most influence on perceived hue.For instance, a typical observer may be able to detect a 0.5 nm shift ina quasi-monochromatic yellowish source, while barely able to detect a 5nm shift in a long-wave red source. But the vision system cannotdiscriminate between a single monochromatic source and (e.g.) a two-tonemetameric source composed of quasi-monochromatic red and green withappropriate balance. This is simply because the red-green opponentsignal can be made to be an identical match. As such, the heightenedsensitivity to monochromatic color shift in the yellow extends todescribing the sensitivity of the balance between the relative amplitudeof a two-tone source. So by transmitting only narrow bands centered onprimary green and primary red, a filter places much greater emphasis onthe detailed structure of the spectral signature of objects and howspecific wavelengths affect the balance between peak (r/g) wavelengthsin determining the perceived hue. And because this occurs in a bandwhere the vision system is most sensitive, large distortions inperceived hue can be introduced.

In reality, it is difficult and even undesirable for a sunglass lens tosample only a narrow band of wavelengths in the red, green and blue.Sunglasses must transmit a certain percentage of lumens according tostandard specifications, which requires broadening the primary colorpass-bands. Moreover, extremely narrow transmission bands can placeexcessive emphasis on specific wavelengths in determining the opponentcolor signal, and thus can cause excessive hue distortion. Accordingly,the bandpass profiles in the embodiments disclosed herein areoscillating or sinusoidal functions, and thus represent a continuousweighted sampling of the M and L curves, with PIF peaks/minima only atsingle wavelengths. The specific balance of the red/green amplitudes inthe presence of a neutral input can be engineered by the placement ofthe yellow minimum wavelength with respect to the M and L curves,selecting the width of the notch (order), and introducing additional LSGfiltration.

On the other hand, filters that do not substantially isolate red andgreen bands, due to shallow transition slopes, insufficient notch width,and low attenuation density, tend to mute the desired chroma enhancementeffect. Insufficient filtering weakens the impact on colorfulness andthe image lacks “pop”. When considering conventional technologies forimplementing plastic sunglass lenses, such as dyes, optimum spectralprofiles for chroma enhancement may be difficult to implement inpractice. However, the present invention permits notch transmissionspectra with sufficiently steep slope, width, and density to clearlydefine the primary color profiles. The yellow notch defines thelong-wavelength transition of the green primary color and theshort-wavelength transition of the red primary color. The cyan notchdefines the short-wavelength transition of the green primary color andthe long-wavelength transition of the blue primary color. Chromatically,the perceived colors can be considered weighted linear combinations ofthese primaries.

The spectral signature of objects we observe in the natural world isgiven by the product of the illuminating (solar) power spectrum and thespectral reflectance of the object. The filter of the present inventionattenuates portions of this signature in a manner that is typically moreselective and higher in optical density than spectral featuresassociated with prevalent objects found in the natural world. Exampleswith relatively low spectral gradient (LSG) include the blue of the sky,browns (e.g. distressed vegetation, dried leaves, wood, and soil),greens and yellow-greens (e.g. healthy vegetation), yellows-ambers (e.g.certain healthy vegetation, straw, certain dried leaves) and reds (e.g.certain vegetation, certain turned-leaves, clay, flagstone). This canalso include synthetic objects, such as brick, house paints, coloredstains, and other pigments that are commonly observed outdoors.Typically, the spectral signature shows significant representation fromall visible wavelengths, along with generally LSG, both of whichcontribute to a moderate/low chroma value. For instance, the brown of atree branch can be associated with a LSG quasi-linear ramp withsignificant contribution from blue through red wavelengths. The green ofthe grass can be a similar LSG ramp with a low-density dip in theorange/red due to chlorophyll absorption. The net result is a drabappearing world; one which lacks colorfulness and differentiation.

For the most part, the natural world consists heavily ofbrown-yellow-green hues of low chroma value. Exceptions to this withhigher chroma signatures are less common, though they are an importantpart of the observed outdoor world; including flowers, fruit/berries,certain turned leaves, and of course high-chroma synthetics, such asstreet/business signs, colorful fabrics, and (e.g.) automotive paints.Such higher-chroma objects typically have HSG transitions and relativelylow representation from certain portions of the visible spectrum. Thepresent invention also affects the appearance of such objects, though ina different way.

In the event that the filters taught herein are desired to have aneutral white-point, objects that appear neutral in the natural worldshould remain substantially so. For objects with any significant chroma,the impact of the filter in distorting the perceived color depends uponthe specific spectral signature. And objects that appear to have thesame color (i.e. metamers) can be affected differently by the filter,when considering the impact of the product of the input spectralsignature with the filter transmission function. Each notch of thefilter can be thought of as a pair of “trim filters”. A yellow notch canbe considered a short-pass trim filter that largely determines primarygreen, and a long-pass trim filter that largely determines primary red.The product of the green trim filter with the spectral signature largelydetermines the perceived green hue and the level of contribution ofprimary green to objects in the image. The product of the red trimfilter with the spectral profile largely determines the perceived redhue and the level of contribution of primary red to objects in theimage. In this case, the only way to perceive yellow in a scene, isthrough the appropriate mixture of red and green primaries establishedby the notch filter.

If for instance the input spectral signature consists of an LSG spanningthe red/green, the perceived hue can be strongly influenced by therelative contribution at the filter peak wavelengths. If however, ahigher chroma input signature of an object has an HSG within the notchregion, the distortion can be very pronounced. Consider an objectsignature (e.g. a flower) with a steep positive spectral gradient thatlies within the yellow notch. The object has relatively weak blue/greenreflectance, with high yellow/red reflectance, appearing (e.g.)yellow-orange. In this instance, the red trim filter may substantiallyeliminate/erase the actual transition of the input signature;red-shifting the hue by eliminating substantially any yellow wavelengthcontribution from the signature. Since the signature contains arelatively weak contribution from shorter wavelengths, the eliminationof yellow causes a significant red hue shift. Visually, theyellow-orange flower may appear a vibrant red color. FIG. 5 shows themeasured spectra for a pink flower, both with and without a filter. Thefilter clearly red-shifts the transition half-power point and increasesthe spectral contrast using high optical density notching.

It should be noted that even objects with already high chroma value canappear to be noticeably further color enhanced when observed through thelens. Consider for example a fluorescent fabric or yellow street sign,which can have a relatively confined and highly efficient spectralreflectance signature. Such objects can appear very vibrant even in theabsence of filtration. The action of the trim filtering can furtherenhance the appearance, giving it a monochromatic, almost self-luminant(or glowing) appearance. Though the filter clearly has a sub-unitytransmission, such objects can actually appear as bright, or evenbrighter due to increased contrast and increased chroma.

As discussed, the filtering herein can be accomplished with virtually nochange in the fidelity of the white-point. It has also been pointed outthat (e.g.) filtered metamers of yellow can be transmitted withvirtually no change in perceived hue or chroma. More broadly, there area range of inputs that can be transmitted with no perceptible increasein chroma. In fact, there may be inputs for which the filter actuallydecreases saturation and mutes color differences. Specifically, it willbe shown that the filter can stretch the range of useful color space insome instances, creating greater color coordinate separation, whilecompressing the separation of color coordinates in other instances. Soit is clearly not in general required or even desired for the filter toenhance the chroma of all objects in order to achieve the overall goalof image chroma enhancement.

In addition to providing cyan and yellow notches for enhanced chroma,filters of the present invention can provide one or more transmissionminima in the high-energy blue (HEB) portion of the spectrum (400-450nm). These wavelengths are known to scatter more readily in the eye,cause fluorescence, are difficult to image onto the retina, and can beharmful to the health of our eyes. Recent research indicates that HEBlight can also disrupt sleep patterns, exacerbated by periods of longexposure to artificial light sources from display devices. HEB lightsuppresses melatonin production, affecting circadian rhythms, which hasbeen linked to certain tumoral diseases, diabetes, obesity anddepression. Conversely, long-wave blue is beneficial both for improvingmood and for color rendering. According to the present invention, thefilter has a transmission peak in the long blue portion of the spectrum(e.g., 455-485 nm) for these reasons. PIF structures are used alone orin combination with other filters, to provide eye-safety and generate adesired visual experience. For instance, an auxiliary UV/blue-blockingfilter can be combined with the HEB blocking filter, to effectivelyattenuate all high-energy blue/UV light (e.g., 400-450 nm). In oneembodiment, a more neutral white point can be achieved by balancing theattenuation of HEB light with the attenuation of a band centered onunique yellow, thereby substantially balancing the b/y opponent signal.

Filter Retardation Analysis

Using the simplest form of the PIF shown in FIG. 2, the design optionsinclude the amount of retardation and the polarizer configuration(parallel/crossed). Retarder and polarizer angles that maximize contrastwere used in this analysis, with the understanding that other angles canhave the effect of reducing optical density, decreasing peaktransmission, or both. To simplify the analysis, the retarder order isarbitrarily selected for a specific green wavelength (e.g., 532 nm),with the order (or number of waves of retardation) varied at thiswavelength. For the crossed polarizer cases, an additional half-wave ofretardation is added to produce a peak at 532 nm. It is to be understoodthat adjustments in green wavelength can be made, which cause shifts inthe other spectral features. The purpose here is to identify a basicrange in retarder order that has the potential to meet the spectralcriteria. Table 1 shows the characteristic features of each spectrum,including high-energy blue (HEB) minimum wavelength, blue peakwavelength, cyan minimum wavelength, green peak wavelength (included forcompleteness), yellow-orange minimum wavelength, and red peakwavelength. Entries with dark shading denote features that fail to meetthe requirements for eye protection and chroma enhancement. Entries withlight shading show features that are at the border-line ofacceptability, and could potentially be made acceptable with a smallshift in retardation. Entries with no shading are well within theacceptable criteria. Results show that a range of 4.0 to 6.0 waves of PCretardation in the green may be preferable, more precisely 5.0 to 5.5waves in the green.

Table 1 shows that there is a range of acceptable solutions, with5.0/5.5 waves being the best. At low order, there is not sufficientretardation dispersion to achieve peaks near the desired (blue/red)primaries and the notches encroach on the blue and spectral orangebands. In particular, the spectral gradient from orange to red isshallow, with red throughput being poor. For retarder order higher thanthe optimum value, the blue peak shifts long of the optimum value, andthe red peak shifts short of the optimum value. A consequence of thelatter is that roll-off on the long red side is significant, so red losscan become an issue. Note that photopic weighting is critical in thisportion of the spectrum when assessing the brightness of red. That is, ashort peak wavelength with poor long-red efficiency can appear as brightas a profile with a longer peak wavelength and broader red profile. Ingeneral, increasing retarder order decreases the width of each pass-bandlobe (often characterized as the full-width-at-half-maximum (FWHM). Thistends to reduce the energy contained in each of the RGB bands, which canreduce brightness, though it also decreases notch width. Narrowing theFWHM also increases the selectivity, which can in principle increasechroma, but given that the profile is sinusoidal, the center wavelengthscannot all be optimized as discussed previously.

The red roll-off that occurs when using (e.g.) 6.0 waves of retardationmay be addressed by substituting a red color polarizer of the prior art,as discussed previously. Specifically, the PE spectrum of the analyzingpolarizer shows a sharp transition near the peak red transmission of thePIF. That is, it behaves as a good polarizer for wavelengths short ofthis, and fails to polarize for longer wavelengths. As discussedpreviously, this can allow a relatively short red peak wavelength, withtransition determined by PIF, while preserving high peak transmissionfor all longer wavelengths.

The optimum solutions shown in Table 1 would perform well for all anglesof incidence if the proper biaxial retarder were used, as discussedpreviously. In the absence of this material, uniaxial solutions must beemployed. Each of the designs of Table 1 has specific spectralcharacteristics, though the basic behavior off-normal is common to everyentry. This includes excellent stability of the transmission spectrum inthe plane of the polarizer (and orthogonal to it), with a large blue/redshift of the spectrum in the ±45 degree azimuth. There is however nosignificant loss in optical density off-normal for any azimuth angle.

Given the commonality in characteristic behavior, the following examplesexamine only one specific design solution in detail: 5.0 waves of greenPC retardation between parallel polarizers. Specific crossed-polarizersolutions are not evaluated in detail. However, it should be noted thatcrossed-polarizer solutions differ somewhat from parallel polarizerconfigurations off-normal due to geometrical distortion. That is,parallel polarizers appear to be parallel at any azimuth, while crossedpolarizers do not in general appear crossed off-normal due togeometrical rotations. This rotation is most extreme in the ±45 degreeazimuth. In general, geometrical rotation of the optic axis of theretarder occurs relative to the polarizer absorption axis, though theeffect is not extreme.

Example 1 Single Layer Uniaxial PC Retarder with Parallel Polarizers

This example is the simplest structure, using a single-layer uniaxial PCfilm between parallel polarizers. It serves as a baseline for theperformance attainable without taking further measures. Table 2 showsoutput characteristics at normal incidence, and at 30 degrees off-normalfor specific key azimuth angles. The L value is the impact of the filtertransmission function on photopic brightness, assuming unpolarized lightand ideal unity transmission polarizers (i.e. sinusoidal contributiononly). As such, the L value with zero retardation is 50%, oscillates atlow order, and converges to 25% at very high order.

The ΔL value is the brightness change off-normal as a percentage of thenormal-incidence value. The color coordinates of the white-point are inx-y space. The shift of the white-point off-normal is given by the usualroot-mean-square difference calculation, relative to the normalincidence white-point coordinate.

The indicator of spectral shift off-normal is taken to be the yellowminimum wavelength value, with the actual shift (Δλ) being that relativeto the normal incidence value. The cyan and yellow transmission valuesare all zero in this case since the optical density is preserved. Theyare included as place markers for direct comparison to laterwide-fielded examples that do not necessarily preserve optical density.

TABLE 1 Single PC retarder PIF solutions versus retarder order at 532nm. Retardation Long @532 nm Polarizer HEB Blue Cyan Yellow/Orange(waves) Orientation Minimum Peak Minimum Green Peak Minimum Red PeakComment 3.0 P 402.6 432.9 475.0 532.0 615.2 >700 Fail 3.5 C 413.9 444.2482.2 532.0 601.4 >700 Fail 4.0 P 424.7 452.4 487.8 532.0 591.6 672.3Fair 4.5 C 432.9 459.6 492.0 532.0 583.9 651.7 Good 5.0 P 402.1/440.1465.8 496.1 532.0 578.3 636.3 Excellent 5.5 C 409.3/446.2 470.4 498.1532.0 573.6 624.5 Excellent 6.0 P 415.9/451.9 475.0 501.7 532.0 570.0615.2 Good 6.5 C 422.1/457.5 478.6 503.3 532.0 566.4 607.5 Fail 7.0 P402.1/423.3/ 482.2 505.3 532.0 563.9 601.9 Fail 462.7

The >25 nm shift (both blue and red) at 30 degree incidence producesmore than a 20% loss in luminance. But more importantly, it causes verylarge color shifts. Given that a just-noticeable-difference (JND) incolor change is about 0.01, the table shows that shifts of more than 10JNDs occur relative to center. The maximum change in color between anytwo points within the FOV is Δxy=0.167.

TABLE 2 Off-normal response of 5.0 wave parallel polarizer uniaxial PCsolution. Incidence Color Yellow Δλ Cyan Yellow Angle Azimuth L (%) ΔL(%) Xy Δxy Min (yellow) Trans (%) Trans (%) 0 — 32.5 — 0.378, — 578.3 —0 0 0.417  30°  0° 32.6 0 0.376, 0.0028 578.8 +0.5 0 0 0.419  30° 45°22.7 −30.0% 0.424, 0.1313 552.6 −25.7 0 0 0.294  30° 90° 32.6 0 0.376,0.0028 578.8 +0.5 0 0 0.419  30° −45°  25.2 −22.5% 0.282, 0.1025 605.0+26.7 0 0 0.381 

Example 2 Wide-Field Filter Using Single Zero-Order HW

The simplest wide-field form of the techniques taught herein is azero-order HW retarder, parallel/perpendicular to the polarizerabsorption axis, between the split multi-order retarders. Thisconfiguration is shown in FIG. 3. This example uses split-retarders,each with a retardation of 2.5 waves at 532 nm. A zero-order HW retarderwith a center wavelength of 535 nm is used to balance the opticaldensity of cyan and yellow minima at normal-incidence. The polarizersare parallel.

Were the HW retarder completely achromatic, the normal incidenceperformance would match closely to that of Table 1. The impact of HWretarder dispersion includes loss in optical density of nulls, loss ofpeak transmission, and incremental spectral shifts, all of whichcontribute to changes in chromaticity. So there is a sacrifice neededeven in normal-incidence spectral performance in order to gainuniformity with incidence angle. The table tracks null center wavelengthand density, but not peak transmission.

The first-order improvement associated with wide-fielding is borne outin much reduced spectral shift in the ±45 degree azimuth, as shown inTable 3. This is at the expense of incremental loss in peak transmissionand optical density, including the normal-incidence case. Moreover, thisinstability is a function of azimuth off-normal as there is anassociated shift in the half-wave retardance center wavelength, which isgreatest in the 0/90 degree azimuth. The rising/falling of opticaldensity in the cyan/yellow notch with azimuth is clearly evident inTable 3. While not captured in this table, such fluctuations in densitycan impact the effectiveness of chroma enhancement. Perhaps mostimportantly however, the spectral shift is much reduced, as is the shiftin white point for all azimuth angles. Relative to the uniaxial case,the color coordinate shift from normal-incidence is a maximum ofΔxy=0.031, or about 3 JNDs. This improves upon the previous example by afactor of 4.2×. The maximum difference between any two points in the FOVis Δxy=0.047, which is reduced by a factor of 3.5×.

The parallel polarizer (normal-incidence) behavior should also becompared to designs using crossed-polarizers, as the impact of thechromatic HW is different. With parallel polarizers, the incrementalchange in HW retardation (with departure from the ideal half-wavewavelength) causes a loss in the optical density of minima, while idealpeak transmission is maintained. The crossed-polarizer version invertsthe spectrum, thus ensuring high density nulls, but with loss in peaktransmission with departure from the half-wave wavelength. Preferencefor high notch density or high peak transmission throughout the visiblemay thus dictate the preferred configuration. While these statements arein general true for normal incidence, both configurations showdependence of peak transmission and optical density off-normal. Whilethe chromatic nature of a zero-order HW retarder compromises theperformance, it does have the benefit of simplicity and does a good jobof suppressing the spectral shift. So if the spectral performance overthe field of view is considered acceptable, this can be a viablesolution.

Example 3 Wide-Field Filter Using Two-Layer Rotator

Another design, shown in FIG. 4, uses a two-layer rotator as a means ofwide-fielding the filter 50. The benefit of increasing the number of HWretarders is that the central polarization transformation can be madeless chromatic over the visible. In this case, two zero-order HW films52 and 54 are arranged between the split retarder (56 and 58), which inturn is between polarizers 60 and 62, as shown in FIG. 4. The two HWretarders have the appearance of effectively rotating the polarizer by90 degrees, so peaks and minima are swapped. This inversion can becounteracted by adding/subtracting a quarter-wave of retardation to eachof the split retarders (denoted by a prime in the figure). To the extentthat the rotator is achromatic over the visible, it appears the same asthe crossed-polarizer case. However, the transformation is notcompletely achromatic so the parallel polarizer case with a quarter-waveof shift added to each outer element (e.g. 2.25 waves), does notcompletely match the crossed polarizer case with no retardation shift(with e.g. 2.5 waves of retardation). The example of Table 4 correspondsto that of FIG. 4, with the split retarders having a retardation of 2.25waves at 532 nm. As shown, the HW retarders are symmetrically placedwith respect to the polarizer, with a center wavelength of 516 nm. Thereis a subtle change in the chromatic behavior of the rotator as the HWangles are rotated symmetrically with respect to the nominal ±22.5°. Achange in the magnitude of the angle causes a shift in the pair ofwavelengths at which the transformation (which can be demonstrated onthe Poincare sphere) is complete. A reduction in angle shifts thewavelengths apart (giving broader wavelength coverage, but with somesacrifice of the mid-band performance), while increasing the anglefurther narrows the spectral coverage. Given the broad range required tocover the visible, some reduction is generally desirable (e.g. the±21.5° example given), while a further increase tends to be less so. Ithas been determined via experimentation and simulations that reasonableresults can be achieved at least in the range of HW angles as low as±19° up to HW angles as high as ±24°.

The addition of a second half-wave retarder enables further improvementin both normal and off-normal characteristics. In this rotator case, thechange in luminance with angle remains very small, but more importantly,the white-point shift is at or below a JND. The optical density is ingeneral high, with transmission remaining below 2%. The only notableloss in performance relative to the previous example, is a 3 nm shift inthe yellow null at one azimuth. Thus, the two-layer example has thebenefit of providing near ideal performance, with the addition of asingle layer.

TABLE 3 Off-normal response of 5-wave parallel polarizer uniaxial PCsolution, with single zero-order HW wide-fielding. Incidence ColorYellow Δλ Cyan Yellow Angle Azimuth L (%) ΔL (%) Xy Δxy Min (yellow)Trans (%) Trans (%) 0 — 33.0 — 0.367, — 577.7 — 2.9 2.2 0.393  30°  0°33.9 2.7 0.375, 0.0187 578.3 +0.6 1.9 7.9 0.410  30° 45° 32.0 3.0 0.369,0.0300 578.8 +1.1 2.2 0.5 0.423  30° 90° 32.9 0 0.354, 0.0191 578.8 +1.14.6 0.1 0.379  30° −45°  32.0 3.0 0.369, 0.0311 578.8 +1.1 2.2 0.50.424 

TABLE 4 Off-normal response of 5.25-wave parallel polarizer uniaxial PCsolution, with two- layer rotator wide-fielding. Incidence Color YellowΔλ Cyan Yellow Angle Azimuth L (%) ΔL (%) xy Δxy Min (yellow) Trans (%)Trans (%) 0 — 32.5 — 0.396, — 573.6 — 0.6 0.2 0.405  30°  0° 32.0 2.70.400, 0.0072 572.1 −1.5 1.3 0.6 0.399  30° 45° 32.5 3.0 0.392, 0.0057574.1 +0.5 1.3 0.9 0.409  30° 90° 32.9 0 0.386, 0.0128 576.7 +3.1 0.21.6 0.413  30° −45°  33.6 3.0 0.392, 0.0057 574.7 +1.1 1.3 0.8 0.409 

Example 4 Wide-Field Filter Using Three-Layer Compound HW Retarder

This example uses the same arrangement of Example 2, but with thezero-order HW retarder replaced by a three-layer compound HW retarder.The HW retarders have a center wavelength of 516 nm. The outer HW layersboth have an orientation of 30 degrees, with the central HW retarderhaving an orientation of −30 degrees. As before, the split retarderseach have a retardation of 2.5 waves at 532 nm.

As Table 5 shows, the addition of a third HW retarder further refinesboth the normal incidence and off-normal incidence performance. Shiftsin brightness are insignificant and the shift in white-point is wellbelow a JND. The improvement over the two-layer solution is incremental,and as such, the question is whether it is significant enough to justifythe additional layer.

TABLE 5 Off-normal response of 5-wave parallel polarizer uniaxial PCsolution, with three-layer compound HW wide-fielding. Incidence YellowΔλ Cyan Yellow Angle Azimuth L (%) ΔL (%) Color xy Δxy Min (yellow)Trans (%) Trans (%) 0 — 32.4 — 0.378, — 578.3 — 0.2 0.1 0.417  30°  0°31.7 −0.7 0.374, 0.0056 578.8 +0.5 0.4 1.3 0.421  30° 45° 32.3 −0.10.374, 0.0044 578.9 +0.6 0.6 0.7 0.419  30° 90° 32.1 −0.3 0.377, 0.0014578.8 +0.5 0.9 0.7 0.416  30° −45°  32.3 −0.1 0.378, 0.0010 578.8 +0.51.1 2.1 0.416 

Specifically, it is important to consider whether this improvement wouldactually be realized in a manufacturing environment. This is because allmodeling results provided in the tables are based on prescribed opticaxis orientation and retardation, with no account being made forpractical issues associated with the as-fabricated material statistics,as well as any changes resulting from fabrication, such as laminationstress, and thermoforming stress. It should be pointed out that MonteCarlo simulations can at times predict lower performance in practicewhen using a greater number of retarder layers with statisticaluncertainty in optic axis and retardation, even though the ideal caseindicates better performance.

The above analysis can be repeated for retarder materials with differentoptical characteristics, such as COP, which has very low birefringencedispersion. A benefit of higher birefringence dispersion is that thedesired filter profile can potentially be achieved with lower retarderorder. Alternatively, materials with even higher birefringencedispersion than PC could potentially further decrease the optimumretarder order. Molecules with controlled birefringence dispersion, suchas the wide-band retarder product (e.g. WB140) developed by Teijin,demonstrate that birefringence dispersion can also be engineered at themolecular level. Table 6 is the zero birefringence dispersioncounter-part to Table 1, which shows that the optimum range of greenretarder order increases to 6-8 waves, potentially 6.5-7.5 waves. Withlower dispersion and higher order, the long-wave red roll-off is more ofan issue unless it is desirable to suppress longer wavelengths. Or, asdiscussed previously, an analyzer with poor PE in the red is used.

TABLE 6 PIF solutions with zero birefringence dispersion retarder.Retardation Long @532 nm Polarizer Blue Cyan Green Yellow/Orange (waves)Orientation HEB Minimum Peak Minimum Peak Minimum Red Peak Comment 5.0 P409.8 443.7 484.3 532.0 591.6 665.1 Fail 5.5 C 418.5 450.9 487.8 532.0585.5 650.2 Fail 6.0 P 426.2 456.0 491.4 532.0 580.3 638.4 Good 6.5 C431.9 461.6 494.0 532.0 576.7 629.1 Excellent, note 691.8 minimum 7.0 P438.5 465.8 496.6 532.0 573.1 620.4 Excellent, note 676.9 minimum 7.5 C444.2 469.9 498.6 532.0 570.6 614.2 Good, note 665.6 minimum 8.0 P406.2/448.3 473.0 501.2 532.0 568.0 608.6 Fair, note 654.8 minimum 8.5 C410.8/452.4 476.0 502.7 532.0 565.4 602.9 Fail, note 646.1 minimum

Examples of Simple Inputs

Based on the above, the HSG notches can cause significant increase inchroma value of natural objects. The product of a low-chroma greenobject (e.g. grass), with the green pass-band profile of the lens, cangive a rich more colorful green. This is accomplished by allowing onlythose wavelengths associated with primary green (of a desired hue) topass. The fundamental limitation however is that low chroma objects alsotend to have broad spectral signatures spanning more than one primecolor, and therefore significant contribution from all three conereceptors. So beyond HEB/cyan/yellow notch filtering, little can be doneto emphasize a particular hue without affecting the hue of all objectsin a scene. A deliberate shift in the balance that affects the lens huemay however be beneficial or desirable in order to obtain the correctrelative emphasis between (e.g.) the redness of objects in a scene andthe greenness of objects in a scene.

The filter seeks to enhance chroma by exploiting an expanded area ofcolor space, both through increased saturation and, increased imagecolorfulness. The latter refers to distorting the hue of certain objectsin a manner that accesses regions of color space not normally observedin a (e.g. outdoor) scene. A particularly dramatic demonstration of thisis the shift of the hue of brownish/reddish objects toward deeper andmore saturated reds. This can change the color space statisticaldistribution, such that areas of color space not normally “populated” inany significant way, are highly represented.

The scan of a long-pass filter, though a simplistic representation of anobject reflectance spectrum, illustrates this point. The reflectancespectrum in this case represents a weak signal for short wavelengths,with a fairly steep transition to a relatively high spectral reflectanceat longer wavelengths. In this example, the reflectance at shortwavelengths is insignificant (about 0.1%), and the reflectance at longwavelengths is unity. The transition bandwidth (10%-90%) averages afairly steep 3.3% of the half-power point (HPP) wavelength. The CIE 1931color space 2-degree color coordinates (from the CIE color space orchromaticity diagram created by the International Commission onIllumination (CIE) in 1931) are tracked both with and without the filteras the HPP of the reflectance spectrum is scanned. Each unique HPP, andassociated color coordinate, can be considered to represent an objectpresent in a scene. The results for the long-pass profile are shown inFIGS. 6a and 6 b.

As the HPP of the reflectance spectrum is red-shifted, the colorinitially shifts from the white-point (i.e. the complete absence offiltering), to a saturated yellow with diminishing of the S-cone signal(i.e. elimination of blue light). As the HPP is scanned to still longerwavelengths, the color coordinate substantially follows the locus of theCIE diagram as the M-cone signal is weakened relative to the L-conesignal, noting that there is virtually no contribution from the S-conesignal throughout this range. The presence of a uniform spectralreflectance (including a strong unique-yellow signal) moderates thecolor coordinate shift. FIG. 6a shows the shift in color coordinate as afunction of HPP wavelength in the absence of the filter, with somespecific color coordinates marked for comparison to the filtered case.As the HPP shifts from short-green (521 nm) to long green (549 nm), thecolor coordinate shifts by Δxy=0.092. This corresponds to an approximateperceived monochromatic color shift from 576 nm to 588 nm, or Δλ=12 nm.

FIG. 6b shows the same scan in HPP wavelength, but with the filterpresent. In this case, the filter corresponds to the 5-wave green PCretarder between parallel polarizers discussed in previous examples. Asbefore, the color shifts from the white point to a saturated yellow butthe distribution of points is different. Because the cyan notchde-emphasizes the HPP in this region, a saturated yellow is achieved ata shorter HPP, while several subsequent color coordinates are compressedand are nearly overlapping. Note that the yellow color coordinate isvery nearly the same as that for the unfiltered case, though it isperceived only through the relative mix of green and red light. But asthe HPP begins to encroach on the short-green, the M-cone signal israpidly diminished, as shown in FIG. 7. The absence of light at greenwavelengths causes a rapid change in the r/g opponent signal, with anassociated hue distortion. As the HPP shifts from short-green (521 nm)to long green (549 nm), the color coordinate shifts by Δxy=0.134. Thiscorresponds to an approximate perceived monochromatic color shift from577 nm to 604 nm, or Δλ=27 nm. In terms of comparison, the ratio ofcolor coordinate shift is 0.134/0.092=1.45×. Moreover, the gain inperceived monochromatic color shift is 27/12=2.25×.

A similar result is obtained when blue-shifting a short-pass profile(inverse of previous example) that represents the object reflectancespectrum. As the HPP of the reflectance spectrum is blue-shifted, thecolor initially shifts from the white-point (i.e. the complete absenceof filtering), toward the cyan with relative diminishing of the L-conesignal. As the HPP encroaches on the green, the M-cone signal isweakened, and the perceived color shifts toward the blue. FIG. 8a showsthe shift in color coordinate as a function of HPP wavelength in theabsence of the filter, with some specific color coordinates marked forcomparison to the filtered case. As the HPP shifts from long-green (549nm) to short green (521 nm), the color coordinate shifts by Δxy=0.233.

FIG. 8b shows the same scan in HPP wavelength, but with the filterpresent. In this case, the filter corresponds to the 5-wave green PCretarder between parallel polarizers discussed in previous examples. Thecolor shifts from the white point toward the cyan, but the separation ofpoints is initially much larger than the unfiltered case (note that thewhite point is different). The lack of yellow light causes a rapid shiftin the r/g opponent signal when red is attenuated. Because of the notchfiltering, the final cyan point is also more saturated than theunfiltered case. With further blue-shifting, the notch filtering causescompression of several subsequent color coordinates, which are nearlyoverlapping. As the HPP shifts from long-green (549 nm) to short-green(521 nm), the color coordinate shifts by Δxy=0.340. In terms ofcomparison, the ratio of color coordinate shift is 0.34/0.233=1.46× theunfiltered case.

As observed above, the notch filtering can produce both increasedcolorfulness and increased saturation. A simple way to illustrateincreased saturation is to consider objects with band-pass reflectancespectral profiles with fixed center wavelength and varying spectralwidth (FWHM). Taking a band-pass (transmission lobe with sine-squaredprofile) to represent an object, the product of the profile with thenotch filter spectrum (in this case 5-waves 532 nm retardation betweenparallel polarizers), gives a modified profile and associated colorcoordinate. FIG. 9a shows the distribution of color coordinates (CIE1931) for the naked-eye perceived colors, and FIG. 9b shows thedistribution of color coordinates observed through the filter. In theabsence of filtering, the broad profile produces a de-saturated greenishhue, which rapidly increases in saturation as the FWHM is narrowed.Below a FWHM of about 50 nm, the increased saturation becomes moreincremental.

As for the filtered case, when the profile is wide the product gives ahigh transmission green lobe, with significant side lobes in theblue/red. Nevertheless, this results in a much more saturated green whenobserving broadband (91 nm FWHM) greenish objects versus the unfilteredcase. When the profile is further narrowed, the side lobe amplitude isgreatly diminished, so the initial color step associated with the 65 nmFWHM is large (though smaller than the unfiltered initial step). Whenthe FWHM is decreased to 52 nm, the side lobes are eliminated and thesaturation is dictated by the green lobe of the notch filter. This stepis larger than that of the unfiltered case. By this third step, anysubsequent decrease in FWHM of the reflectance spectrum has relativelylittle impact on color, due to the compression of color coordinatesfound previously. Conversely, an object must have a reflectance spectralwidth of approximately 35 nm (6th point on FIG. 8a ) to achieve the samelevel of saturation as the filtered case with a FWHM of 52 nm.

The purpose of optimized filtration is to selectively reduce colors thatare considered unimportant, or even reduce visual performance, whilemore efficiently transmitting colors that are important for vision. Anapproach to this optimization is to maximize ΔE for a set of inputspectral power distributions (SPDs). A color difference between twoobjects can be calculated using formulas such asΔE*=√{square root over ((L* ₂ −L* ₁)²+(a* ₂ −a* ₁)²+(b* ₂ −b* ₁)²)}

which is influenced by the difference in lightness (L*₂−L*₁) andopponent color signals ((a*₂−a*₁) and (b*₂−b*₁)) between any two objects(represented by the subscripts 1 and 2). For example, a color differencebetween a tennis ball and the court in bright sunlight can be determinedin the absence of filtering using the above equation. With a filterinserted, the calculation can be repeated. If ΔE increases, then thefilter can be considered to enhance the contrast. Note that anormalization is required to account for the difference in lightnessbetween the filtered and unfiltered light levels when comparing colordifferences. Alternatively, color differences can be calculated usingonly color coordinate shifts, or hue/chroma shifts.

The outcome of such calculations depends upon lighting conditions(level, spectral luminance and angular distributions), the BRDF of theset of salient objects selected, and the characteristics of the viewer'svision system. For example, a down-hill skier with normal color visionmay well prefer a filter that is quite different than that preferred bya golfer. And a filter that is desirable to an individual with colorvision deficiency may be objectionable to one with normal color vision.

Consider a filter that does not attempt to improve the M/L opponentsignal (perhaps even suppressing it), but concentrates specifically onthe S/(M+L) opponent signal. This may be beneficial in certain scenecontexts for a normal trichromat, and can also be preferred for adichromat, or anomalous trichromat more generally.

Example: Maximize Image Quality for a Protanope

Assumptions:

The L cone is completely missing in both eyes, and there is notrichromatic vision in the fovea or at larger angles.

The subject confuses colors of equal brightness along the green-redportion of the CIE diagram locus.

The subject cannot discriminate between any color of equal brightnessalong any confusion line.

The total number of colors that a Protanope can discriminate is 17(versus 150 for a normal trichromat). The confusion lines of a protanopeconverge to the point (x=0.747, y=0.253). Colors furthest from thispoint provide the greatest sensitivity to shifts in hue, particularly inthe cyan. Maximum sensitivity occurs approximately at the neutralopponent response between the S and M cone fundamentals. The range of480-515 nm is an important spectral band for color discrimination.Shifts in hue along a confusion line toward greater chroma (toward thecyan portion of the CIE locus) are beneficial. These shifts creategreater separation between points on adjacent confusion lines andtherefore better color discrimination. Protanopes are most sensitive towavelength shift at about 495 nm, and can resolve a shift of about 2-3nm. This sensitivity decays rapidly for departures from the peak. Bycontrast, a normal trichromat can resolve about 1 nm at 495 nm, and atleast 2 nm throughout most of the visible.

Consider a filter delivering a two-peak transmission function, as shownin FIG. 13. It corresponds to a 4.0 wave polycarbonate retarder at 440nm between parallel polarizers. The peak center wavelengths correspondapproximately to peaks in the S and M cone fundamentals. The nullbetween these wavelengths is in the long-wave blue, approximately wherethe neutral opponent response is. By greatly reducing the transmittedlight near the neutral wavelength (near spectral cyan), the perceivedhue is more dependent upon the relative balance between short-wave blueand green content in the input SPD. This tends to push the filtered SPDof natural objects closer to the locus, where color discrimination isbetter, while potentially distorting the hue of many objects. Whilenarrow-band spectral-cyan objects would tend to become much darker, theoccurrence of such objects in the natural world is very rare.

The Protanope sensitivity to hue shift at/near the green-red portion ofthe CIE locus is weak. It coincides with a confusion line due to absenceof the M/L opponent signal. So, apart from selecting the peak centerwavelengths to push the input SPD as far as possible toward the leftportion of the locus, little can be done except to control perceivedlightness. The peak achromatic sensitivity to brightness is givenessentially by the M curve, which peaks at 542 nm. It decays to 50% at502/586 nm, to 10% at 450/620 nm, and to 1% at 408/653 nm. So aProtanope is very sensitive to the brightness of green objects, with aresponse that is relatively similar to normal trichromats at shorterwavelengths, but is much different at longer wavelengths. With theabsence of the L cone, longer-wave red objects appear much darker thanthey do for normal trichromats. A yellow object is perceived as onlyhalf the brightness of a green object of the same lightness, and anorange-red object is only 10% of the brightness of a green object of thesame lightness. While the Protanope cannot discern any hue difference,they can exploit the achromatic pathway to discriminate between objectsof similar lightness along the red/green locus. This of course comeswith some ambiguity, since the lightness of an object is variable. Sooptimizing the green-red transmission profile requires some knowledge ofthe input data set of salient objects. The combination of the appearanceof familiar objects with their lightness is another way that dichromatscan discriminate color. For instance, a Protanope knows that a ripetomato is red and an under-ripe tomato is green, and can assume that thedarker tomato is relatively ripe.

Given the lack of color sensitivity, it may be beneficial to use afilter that controls the taper of the transmission throughout thegreen-red region to enhance contrast between more frequently occurringcolors (e.g.) of the natural world. Given that the M cone provideslittle information past 653 nm, the filter can strongly attenuate longerwavelengths, in order to gain better discrimination in a band of shorterwavelengths that provide a greater richness of information. If theobjective is to establish correspondence between brightness and color,it is also important to have an understanding of the lightnessstatistics of the input data set. The spectrum of FIG. 13 shows such ataper, with a 50% point of 579 nm, 10% at 605 nm, and 1% at 618 nm. Byholding transmission high at the peak of the M curve, and tapering it toa minimum at 626 nm, the contrast sensitivity is enhanced in a keyspectral range for natural objects. The consequence is some compressionof the contrast at longer wavelengths, where longer-wavelength reds areall perceived as very dark. In practice, a PIF-based filter can becombined with the tint and wavelength dependent polarizing efficiency(also known as chromatic polarizing efficiency) of a polarizer tocontrol the detailed taper of the transmission in the green-red.

With the peak center wavelengths selected, low spectral gradientfiltering can be applied to adjust the relative amplitude of the blueand green peaks, by (e.g.) using a tinted polarizer. This can be done toselect the neutral-point of the lens, and should have little impact oncolor discrimination. The line connecting the two peak centerwavelengths of the filter on the CIE diagram, of optimum ratio, canyield a neutral point that may be undesirable to a normal trichromat,because it may appear too cyan. Since Protanopes will accept cyan huesas matches for white, there is significantly more freedom in selectinglens hue. The native color of the spectrum of FIG. 13 is x=0.248,y=0.416, which is fairly cyan/green for a normal trichromat, and is evenabove the neutral confusion line of a Protanope. As such, it may bebeneficial to attenuate the green peak relative to the short-blue peakin order to shift the color coordinate to an intersection point of theneutral confusion line. This assumes that the objective is to obtain aneutral hue. Other lens hues can of course be selected to shift thecolor from the native hue to one that may be desired to satisfy certainviewer preferences. Moreover, a filter with blue and yellow peaks can beadjusted to produce a neutral white point as perceived by a normaltrichromat. While general color discrimination of such a filter isnon-optimum, in certain scene contexts it may be preferred.

Filters of the present invention need not have a color-balancedtransmission function. As discussed above, a filter for CVD can have anacceptable white point that may not be desirable for one with normalcolor vision. Also, a filter could have a transmission function thatcompensates for the environment to create a more pleasing white point.This could apply to scuba diving or snorkeling, where the water filtersthe sunlight to effectively modify the illuminant. Similarly,task-specific filters may assign greater weight to resolving aparticular color difference at the expense of white point. An example ofthis is a filter used in surgery (e.g. O2-Amp).

Lens Fabrication

Chroma enhancement eyewear lenses as taught herein can be fabricatedusing many of the same processes as more conventional polarizing lenses.The lens may be a stand-alone chroma enhanced sunglass filter, as shownusing the laminate of FIG. 10. Alternatively, it may be a clip-onpolarizing filter that produces chroma enhancement as a stand-alonefilter, or when combined with another polarizing lens, as illustrated inthe laminate of FIG. 11. In the event that the latter relies uponpolarization from a separate lens, protective substrates on either sideof the clip-on filter must preserve the state of polarization.

The high throughput fabrication of robust and flexible retarder stackswith low transmitted wavefront distortion (i.e. no non-uniform adhesivethickness issues), is best done using solvent bonding. For PC, this isbest done using a ketone, which yields a reliable bond with small lossin retardation. Similar solutions also exist for olefin based retarders.The retarder stack can then be inserted into a functional filter stackas shown. This includes a low birefringence outer substrate to preservethe incident state of polarization (convex surface). Alternatively, ahighly stretched substrate with optic axis parallel/perpendicular to thepolarizer absorption axis can be used. Such products using polycarbonateand polyester are readily available (e.g. Mitsubishi Gas and Chemical,and Toyobo). In the event that a non-polarizing lens is desired, aquarter-wave retarder or highly stretched retarder can be added to thestack oriented at 45 degrees to the polarizer. This layer can eitherfunction as the outer substrate or it can be positioned between theouter substrate and the input polarizer. The polarizers can be anymaterial used for conventional polarizing sunglasses, ideally using onlythe functional PVA layer (e.g. no bounding triacetyl cellulose (TAC)layers). The inner substrate can exhibit birefringence, as the SOP isalready analyzed at this point. In some instances, injection molding orcasting onto this substrate is required (for e.g. prismatic correction),so the material may need to be compatible with such a process.

Substrate optical preferences include high refractive index(preferably >1.5), low haze, low retardation (or controlledretardation), no crazing from post processes, and high optical surfacequality. Some absorption of light (e.g. HEB) may be acceptable or evenpreferred to the extent that it facilitates meeting filteringobjectives. Mechanical preferences include impact resistance,dimensional stability, flexibility, low weight, compatibility withthermoforming (Tg), compatibility with adhesive processes (e.g. highsurface energy), compatibility with surface coatings (e.g. hard-coats),and reliability in harsh environments (e.g. high temperature, flux andhumidity).

A final laminated lens stack, including outer/inner protectivesubstrates, first/second polarizers, and the retarder stack, can bebonded using (e.g.) an optically clear thermoset urethane adhesive. Anyadhesive that yields the desired optical, mechanical, and durabilityrequirements, which is manufacturing friendly, is viable. This includesresins of acrylic, polyester, epoxy, silicone, or melamine.Manufacturing processes can use various cure methods, includingthermosets, radiation cures, PSAs, two-part reactions, B-stage, andcyanoacrylates.

The laminated stack can be die cut into finished part geometry for flator one-axis curved product (e.g. lens filters, goggles or shields), orit can be cut into an appropriate standard geometry for furtherprocessing. A filter can be subsequently laminated between rigidsubstrates (e.g. glass) for certain applications. For eyewear lenses,the geometry is typically a circular disk, but it can also be of othergeometries, such as elliptical, or a rectangular. A disk can bethermoformed into a blank of appropriate 3D geometry, as illustrated inFIG. 12, using conventional processes. This may include spherical (ofvarious base-curvature), aspheric, toroidal, or elliptical. A formedblank can also be bonded to another element for further processing intoprescription lenses. A blank may also be press-polished, and/or receivea particular desired spatial distribution of thickness for improvedvisual quality (e.g. prismatic correction). This process may includeinjection molding on the inner surface, or casting of a resin oneither/both surfaces. Additional functional coatings, such ashard-coats, antireflection coatings, (isotropic or patterned) mirrorcoatings, dielectric filter coatings, hydro-phobic coatings, andoleo-phobic coatings can be applied in the usual fashion. These blankscan then be processed into finished lenses/eyewear using conventionalprocesses.

For lens blanks using an injection/casting (overmolding) process, a thinfunctional blank can be fabricated with material subsequently cast oneither side of it. A four layer retarder stack is typically 280 micronsthick, though a thickness of 160 microns is also feasible. Givenadhesive and PVA thicknesses, and perhaps a thin highly stretched layerencapsulating the functional structure, the overall blank thickness maybe 600 microns or less. Such a wafer can be formed and then insertedinto a casting process to form a lens blank of appropriate thickness.

The present invention describes a color enhancing polarizationinterference filter that can be used for visual enhancement in front ofthe eye, or at any appropriate distance that accomplishes the samefunction. It can also be used with other types of (electronic) imageforming devices. In such a display system 100, shown in FIG. 14, afilter 102 can be placed between a light source 104 and a display 106,or it can be placed over a display device (on the side of the displayviewed by the user), to modify the SPD for either backlit, self-emissive(e.g. OLED), or passive illuminated display devices. Benefits of suchfiltration can include improved color gamut, increased contrast, orambient light rejection. For instance, a filter matched to a display SPDwill pass lumens efficiently, while more strongly rejecting the SPDassociated with illuminating glare.

The filter of the present invention can also be used in an imagingsystem 120 by placement in front of image capturing media (e.g. film),electronic sensor 122, or sensor array, as shown in FIG. 15. The system120 is shown to include a lens 124, a color enhancement filter 126 (asdescribed throughout), the sensor 122, and a processor 128. It will beunderstood that the 120 is shown and described herein in simplifiedfashion with many details and components omitted for ease ofillustration. The function of the PIF is to assign weighting toparticular wavelengths within the captured SPD. As in the visual case,such filtering can enhance colors, improve detection of boundaries, andincrease the signal-to-noise ratio when properly optimized. An exampleof this can be camouflage detection.

The oscillating transmission functions achieved by the optical filterstaught herein typically oscillate with unity amplitude and frequencydetermined by the “order” (number of waves of phase-difference) of theretarder. As shown in FIG. 1, this may mean that each of themaximums/peaks in the transmission curve are at the same amplitude andwhich may be at or near 100% transmission at that wavelength. Also, eachof the minimums/valleys in the transmission curve are at the sameamplitude and which may be at or near 0% transmission at thatwavelength. In actuality, these valleys may be at or near 20% or lessthan the maximums, at or near 15% or less than the maximums, at or near10% or less than the maximums, or at or near 5% or less than themaximums. As can be seen, in each of the conditions described in theprevious sentence, the transmission at the peaks will be greater thanfour times the transmission at the valleys and in some of thecases/conditions described in the previous sentence, the transmission atthe peaks will be greater than ten times the transmission at thevalleys.

Further, the oscillating transmission functions may have a period thatincreases with increasing wavelength (due to the inverse wavelength, andΔn varying with wavelength as discussed above). Thus, when thetransmission function is plotted versus wavelength (as shown in FIG. 1),the adjacent maximums are closer together (in wavelength) on the shorterend of the wavelength spectrum and further apart (in wavelength) on thelonger end of the wavelength spectrum. This effect can be used by thedesigner in selecting desired peaks and valleys in the transmissionspectrum. Thus, as shown in FIG. 1, peaks can be provided in the red andgreen with a valley in the yellow/orange, and a peak can be provided inthe long-wave blue with valleys in cyan and high-energy blue. As aconsequence of the nature of the oscillating transmission function ofthe filters disclosed herein, there will be additional minimums andmaximums outside of the visible wavelength range. As but one example,most of the filters will have at least one minimum between 700 nm and1400 nm. As an alternative to the peaks and valleys shown in FIG. 1, asshown in the Protanope Filter of FIG. 13, peaks can be provided in theblue and green with valleys in the high-energy blue, cyan, and red.

Another way of describing the net effect achieved by the optical filtersusing PIFs with uniaxial retarders (where N=2) to create oscillatingtransmission functions is that a passive filter is provided that passesdesired (relatively-narrow) wavelength ranges, such as the wavelengthranges of the three primary colors (red, green, and blue).

It is believed that past use of PIFs emphasized the need to have N equalto 3 or greater in order to avoid the non-desirable sinusoidaltransmission functions that would result from N=2. For example,double-notch PIFs have been proposed with 7 layers of retarders, tocreate N=8. However, with wide-fielding, such a PIF would require aminimum of 21 layers and be complex and expensive.

One of the motivations of developing the optical filters disclosedherein was seeking to design a color enhancement filter based on the useof inexpensive uniaxial film. The present invention accomplishes thiswith a simple filter construction capable of (e.g.) notching in the HEB,spectral cyan, and spectral yellow using a single oscillatoryinterference, which can also preserve neutrality and color uniformitywith incidence angle. Also, without regard to uniaxial film, there hasbeen a long-felt need for a color enhancement filter that was simple andinexpensive. Further, there has been a long-felt need for a colorenhancement filter that produced the desired wavelength ranges andfiltered out the undesired wavelength ranges. Others tried to achieve aselective filtering of wavelength ranges with dye filters.Unfortunately, such filters do not produce sufficiently low minimums atthe optimum center wavelengths.

While the embodiments of the invention have been illustrated anddescribed in detail in the drawings and foregoing description, suchillustration and description are to be considered as examples and notrestrictive in character. For example, certain embodiments describedhereinabove may be combinable with other described embodiments and/orarranged in other ways (e.g., process elements may be performed in othersequences). Accordingly, it should be understood that only exampleembodiments and variants thereof have been shown and described.

I claim:
 1. An optical filter, comprising: a polarization interferencefilter (PIF), including at least: an input polarizer; one or moreretarders; and an output polarizer; wherein the PIF has a transmissionacross the 400 nm to 700 nm spectrum that is substantially sinusoidalhaving minimums and maximums, and with a single maximum in the 610 nm to680 nm range, a single maximum in the 520 nm to 555 nm range, and atleast one maximum in the 455 nm to 480 nm range, and with a singleminimum in the 485 nm to 515 nm range, a single minimum in the 565 nm to590 nm range, and at least one minimum in the 400 nm to 450 nm range. 2.An optical filter, including: an input polarizer; an input chromaticretarder; an output chromatic retarder; a neutral Polarization ControlUnit (PCU) therebetween, including: one or more zero-order half-waveuniaxial retarders, wherein the polarization transformation by the PCUcreates a net chromatic retardation that is substantially independent ofangle-of-incidence; and an output polarizer; wherein the filter has atransmission across the 400 nm to 700 nm spectrum with a single maximumin the 610 nm to 680 nm range, a single maximum in the 520 nm to 555 nmrange, and at least one maximum in the 455 nm to 480 nm range, and witha single minimum in the 485 nm to 515 nm range, a single minimum in the565 nm to 590 nm range, and at least one minimum in the 400 nm to 450 nmrange.
 3. An optical filter as defined in claim 2, wherein the input andoutput polarizers have parallel axes of polarization; wherein thechromatic retarders are composed of polycarbonate and each have aretardance (Γ) of 2.25 waves in the 505 nm to 555 nm range; and whereinthe PCU provides rotation about an axis in the range of −19 to −24degrees and an axis in the range of +19 to +24 degrees.
 4. An opticalfilter as defined in claim 2, wherein the PCU includes an odd number ofhalf-wave retarders.
 5. An optical filter as defined in claim 4, whereinthe PCU is a single half-wave retarder with the slow and fast axes ofthe retarder aligned at 0 degrees and 90 degrees with the orientation ofa polarization axis of the input polarizer.
 6. An optical filter asdefined in claim 4, wherein the PCU is a compound half-wave retarderwith three retarder layers and with the fast axis of: a first of thethree retarder layers oriented at −30 degrees with an orientation of apolarization axis of the input polarizer, a second of the three retarderlayers oriented at +30 degrees with the orientation of the polarizationaxis of the input polarizer, and a third of the three retarder layersoriented at −30 degrees with the orientation of the polarization axis ofthe input polarizer.
 7. An optical filter, comprising: a polarizationinterference filter (PIF), including at least: an input polarizer; aretarder stack that generates only two output impulses from a polarizedimpulse input; and an output polarizer; wherein the PIF has atransmission across the 400 nm to 700 nm spectrum with a single maximumin the 610 nm to 680 nm range, a single maximum in the 520 nm to 555 nmrange, and at least one maximum in the 455 nm to 480 nm range, and witha single minimum in the 485 nm to 515 nm range, a single minimum in the565 nm to 590 nm range, and at least one minimum in the 400 nm to 450 nmrange.
 8. An optical filter as defined in claim 7, wherein asubstantially neutral white-point is achieved.
 9. An optical filter asdefined in claim 7, wherein each of the minimums in the transmissionspectrum is less than 20% of each of maximums.
 10. An optical filter asdefined in claim 7, wherein there is at least one minimum in the 700 nmto 1400 nm range.
 11. An optical filter as defined in claim 7, whereinat least one retarder in the retarder stack includes solvent bondedretarder films.
 12. An optical filter as defined in claim 7, wherein atleast one retarder in the retarder stack includes at least one uniaxialstretched retarder film.
 13. An optical filter as defined in claim 12,wherein the film includes polycarbonate.
 14. An optical filter asdefined in claim 12, wherein the film includes polyolefin.
 15. Anoptical filter as defined in claim 7, wherein at least one of thepolarizers include a chromatic dye that controls a color balance andoverall hue of the optical filter.
 16. An optical filter as defined inclaim 7, wherein the optical filter is arranged as a laminated stack oflayers.
 17. An optical filter as defined in claim 16, wherein thepolarizers include polyvinyl alcohol.
 18. An optical filter as definedin claim 16, wherein the retarder stack is formed on glass.
 19. Anoptical filter as defined in claim 16, wherein the retarder stack isformed on isotropic plastic.
 20. An optical filter as defined in claim16, wherein each of the layers of the laminated stack lie substantiallyin their own plane.
 21. An optical filter as defined in claim 16,wherein each of the layers of the laminated stack is curved.
 22. Anoptical filter as defined in claim 7, wherein each of the components ofthe optical filter is thermoformed.
 23. An optical filter as defined inclaim 7, wherein the optical filter is arranged for use by a personhaving color vision deficiency.
 24. An optical filter as defined inclaim 7, wherein the optical filter is arranged for use as a sunglasslens.
 25. An optical filter as defined in claim 7, wherein the opticalfilter is arranged for filtering light before it is capturedelectronically.
 26. An optical filter as defined in claim 7, wherein theoptical filter is arranged as part of a display that displays images.27. An optical filter as defined in claim 7, wherein the optical filteris arranged for filtering light provided to a human performing anactivity.